资源说明：Small Stack-Based Computer Compiler -- Verilog micro controller for FPGA housekeeping with peripherals

SSBCC.9x8 is a free Small Stack-Based Computer Compiler with a 9-bit opcode,
8-bit data core designed to facilitate FPGA HDL development.

The primary design criteria are:
- high speed (to avoid timing issues)
- low fabric utilization
- vendor independent
- development tools available for all operating systems

It has been used in Spartan-3A, Spartan-6, Virtex-6, and Artix-7 FPGAs and has
been built for Altera, Lattice, and other Xilinx devices.  It is faster and
usually smaller than vendor provided processors.

The compiler takes an architecture file that describes the micro controller
memory spaces, inputs and outputs, and peripherals and which specifies the HDL
language and source assembly.  It generates a single HDL module implementing
the entire micro controller.  No user-written HDL is required to instantiate
I/Os, program memory, etc.

The features are:
- high speed, low fabric utilization
- vendor-independent Verilog output with a VHDL package file
- simple Forth-like assembly language (43 instructions)
- single cycle instruction execution
- automatic generation of I/O ports
- configurable instruction, data stack, return stack, and memory utilization
- extensible set of peripherals (I2C busses, UARTs, AXI4-Lite busses, etc.)
- optional interrupt peripheral and interrupt handler
- extensible set of macros
- memory initialization file to facilitate code development without rebuilds
- simulation diagnostics to facilitate identifying code errors
- conditionally included I/Os and peripherals, functions, and assembly code

SSBCC has been used for the following projects:
- operate a media translator from a parallel camera interface to an OMAP GPMC
  interface, detect and report bus errors and hardware errors, and act as an
  SPI slave to the OMAP
- operate two UART interfaces and multiple PWM controlled 2-lead bi-color LEDs
- operate and monitor the Artix-7 fabric in a Zynq system using AXI4-Lite
  master and slave buses, I2C buses for timing-critical voltage measurements
- coordinate data from multiple low data rate AXI-Streams and write them to the
  processor memory through an AXI4-Master port

The only external tool required is Python 2.7.


DESCRIPTION
================================================================================

The computer compiler uses an architectural description of the processor stating
the sizes of the instruction memory, data stack, and return stack; the input and
output ports; RAM and ROM types and sizes; and peripherals.

The instructions are all single-cycle.  The instructions are:
- 4 arithmetic instructions:  addition, subtraction, increment, and decrement
- 2 carry bit instructions:  +c and -c for addition and subtraction respectively
- 3 bit-wise logical instructions:  and, or, and exclusive or
- 7 shift and rotation instructions: <<0, <<1, 0>>, 1>>, <>, and lsb>>
- 4 logical instructions:  0=, 0<>, -1=, -1<>
- 6 Forth-like data stack instructions:  drop, dup, nip, over, push, swap
- 3 Forth-like return stack instructions:  >r, r>, r@
- 2 I/O: inport, outport
- 6 memory read and write with optional address post increment or post decrement
- 2 jump and conditional jump
- 2 call and conditional call
- 1 function return
- 1 nop

The 9x8 address space is up to 8K.  This is achieved by pushing the 8 lsb of the
target address onto the data stack immediately before the jump or call
instruction and by encoding the 5 msb of the address within the jump or call
instruction.  The instruction immediately following a jump, call, or return is
executed before the instruction sequence at the destination address is executed
(this is illustrated later).

Up to four banks of memory, either RAM or ROM, are available.  Each of these can
be up to 256 bytes long, providing a total of up to 1 kB of memory.

The assembly language is Forth-like.  Built-in macros are used to encode the
jump and call instructions and to encode the 2-bit memory bank index in memory
store and fetch instructions.

The computer compiler and assembler are written in Python 2.7.  Peripherals are
implemented by Python modules which generate the I/O ports and the peripheral
HDL.

The computer compiler is documented in the doc directory.  The 9x8 core is
documented in the core/9x8/doc directory.  Several examples are provided.

The computer compiler and assembler are fully functional and there are no known
bugs.


SPEED AND RESOURCE UTILIZATION
================================================================================

These device speed and resource utilization results are copied from the build
tests.  The full results are listed in the core/9x8/build directories.  The
tests use a minimal processor implementation (clock, reset, and one output).
Device-specific scripts state how these performance numbers were obtained.

VENDOR          DEVICE          BEST SPEED      SMALLEST RESOURCE UTILIZATION
------          ------          ----------      -------------------------------
Altera          Cyclone-III     190.6 MHz       282 LEs           (preliminary)
Altera          Cyclone-IV      192.1 MHz       281 LEs           (preliminary)
Altera          Stratix-V       372.9 MHz       198 ALUTs         (preliminary)
Lattice         LCMXO2-640ZE-3   98.4 MHz       206 LUTs          (preliminary)
Lattice         LFE2-6E-7       157.9 MHz       203 LUTs          (preliminary)
Xilinx          Artix-7         TBD             163 slice LUTs (48 slices)
Xilinx          Kintex-7        TBD             158 slice LUTs (44 slices)
Xilinx          Spartan-3A      149.4 MHz       232 4-input LUTs (129 slices)
Xilinx          Spartan-6       193.7 MHz       124 Slice LUTs (34 slices)
Xilinx          Virtex-6        275.7 MHz       122 Slice LUTs (38 slices) (p.)

Disclaimer:  Like other embedded processors, these are the maximum performance
claims.  Realistic implementations will produce slower maximum clock rates,
particularly with lots of I/O ports and peripherals and with the constraint of
existing with other subsystems in the FPGA fabric.  What these performance
numbers do provide is an estimate of the amount of slack available.  For
example, you can't realistically expect to get 110 MHz from a processor that,
under ideal conditions, places and routes at 125 MHz, but you can with a
processor that is demonstrated to place and route at 150 MHz.


EXAMPLE:
================================================================================

The LED flasher example demonstrates the simplicity of the architectural
specification and the Forth-like assembly language.

The architecture file, named "led.9x8", with the comments and user header
removed, is as follows:

  ARCHITECTURE    core/9x8 Verilog

  INSTRUCTION     2048
  RETURN_STACK    32
  DATA_STACK      32

  PORTCOMMENT LED on/off signal
  OUTPORT 1-bit o_led O_LED

  ASSEMBLY led.s

The ARCHITECTURE configuration command specifies the 9x8 core and the Verilog
language.  The INSTRUCTION, RETURN_STACK, and DATA_STACK configuration commands
specify the sizes of the instruction space, return stack, and data stack.  The
content of the PORTCOMMENT configuration command is inserted in the module
declaration -- this facilitates identifying signals in micro controllers with a
lot of inputs and outputs.  The single OUTPORT statement specifies a 1-bit
signal named "o_led".  This signal is accessed in the assembly code through the
symbol "O_LED".  The ASSEMBLY command specifies the single input file "led.s,"
which is listed below.  The output module will be "led.v"

The "led.s" assembly file is as follows:

  ; Consume 256*5+4 clock cycles.
  ; ( - )
  .function pause
    0 :inner 1- dup .jumpc(inner) drop
  .return

  ; Repeat "pause" 256 times.
  ; ( - )
  .function repause
    0 :inner .call(pause) 1- dup .jumpc(inner) drop
  .return

  ; main program (as an infinite loop)
  .main
    0 :inner 1 ^ dup .outport(O_LED) .call(repause) .jump(inner)

This example is coded in a traditional Forth structure with the conditional
jumps consuming the top of the data stack.  Examining the "pause" function, the
".function" directive specifies the start of a function and the function name.
The "0" instruction pushes the value "0" onto the top of the data stack.
":inner" is a label for a jump instruction.  The "1-" instruction decrements the
top of the data stack.  "dup" is the Forth instruction to push a duplicate of
the top of the data stack onto the data stack.  The ".jumpc(inner)" macro
expands to three instructions as follows:  (1) push the 8 lsb of the address at
"inner" onto the data stack, (2) the conditional jump instruction with the 5 msb
of the address of "inner" (the jumpc instruction also drops the top of the data
stack with its partial address), and (3) a "drop" instruction to drop the
duplicated loop count from the top of the data stack.  Finally, the "drop"
instruction drops the loop count from the top of the data stack and the
".return" macro generates the "return" instruction and a "nop" instruction.

The function "repause" calls the "pause" function 256 times.  The main program
body is identified by the directive ".main"  This function runs an infinite loop
that toggles the lsb of the LED output, outputs the LED setting, and calls the
"repause" function.

A tighter version of the loop in the "pause" function can be written as

  ; Consume 256*3+3 clock cycles.
  ; ( - )
  .function pause
    0xFF :inner .jumpc(inner,1-) .return(drop)

which is 3 cycles long for each iteration, the "drop" that is normally part
of the ".jumpc" macro has been replaced by the decrement instruction, and the
final "drop" instruction has replaced the default "nop" instruction that is
normally part of the ".return" macro.  Note that the decrement is performed
after the non-zero comparison in the "jumpc" instruction.

A version of the "pause" function that consumes exactly 1000 clock cycles is:

  .function pause
    ${(1000-4)/4-1} :inner nop .jumpc(inner,1-) drop .return

The instruction memory initialization for the processor module includes the
instruction mnemonics being performed at each address and replaces the "list"
file output from traditional assemblers.  The following is the memory
initialization for this LED flasher example.  The main program always starts at
address zero and functions are included in the order encountered.  Unused
library functions are not included in the generated instruction list.

  reg [8:0] s_opcodeMemory[2047:0];
  initial begin
    // .main
    s_opcodeMemory['h000] = 9'h100; // 0x00
    s_opcodeMemory['h001] = 9'h101; // :inner 0x01
    s_opcodeMemory['h002] = 9'h052; // ^
    s_opcodeMemory['h003] = 9'h008; // dup
    s_opcodeMemory['h004] = 9'h100; // O_LED
    s_opcodeMemory['h005] = 9'h038; // outport
    s_opcodeMemory['h006] = 9'h054; // drop
    s_opcodeMemory['h007] = 9'h10D; // 
    s_opcodeMemory['h008] = 9'h0C0; // call repause
    s_opcodeMemory['h009] = 9'h000; // nop
    s_opcodeMemory['h00A] = 9'h101; // 
    s_opcodeMemory['h00B] = 9'h080; // jump inner
    s_opcodeMemory['h00C] = 9'h000; // nop
    // repause
    s_opcodeMemory['h00D] = 9'h100; // 0x00
    s_opcodeMemory['h00E] = 9'h119; // :inner 
    s_opcodeMemory['h00F] = 9'h0C0; // call pause
    s_opcodeMemory['h010] = 9'h000; // nop
    s_opcodeMemory['h011] = 9'h05C; // 1-
    s_opcodeMemory['h012] = 9'h008; // dup
    s_opcodeMemory['h013] = 9'h10E; // 
    s_opcodeMemory['h014] = 9'h0A0; // jumpc inner
    s_opcodeMemory['h015] = 9'h054; // drop
    s_opcodeMemory['h016] = 9'h054; // drop
    s_opcodeMemory['h017] = 9'h028; // return
    s_opcodeMemory['h018] = 9'h000; // nop
    // pause
    s_opcodeMemory['h019] = 9'h100; // 0x00
    s_opcodeMemory['h01A] = 9'h05C; // :inner 1-
    s_opcodeMemory['h01B] = 9'h008; // dup
    s_opcodeMemory['h01C] = 9'h11A; // 
    s_opcodeMemory['h01D] = 9'h0A0; // jumpc inner
    s_opcodeMemory['h01E] = 9'h054; // drop
    s_opcodeMemory['h01F] = 9'h054; // drop
    s_opcodeMemory['h020] = 9'h028; // return
    s_opcodeMemory['h021] = 9'h000; // nop
    s_opcodeMemory['h022] = 9'h000;
    s_opcodeMemory['h023] = 9'h000;
    s_opcodeMemory['h024] = 9'h000;
    ...
    s_opcodeMemory['h7FF] = 9'h000;
  end


DATA and STRINGS
================================================================================

Values are pushed onto the data stack by stating the value.  For example,

  0x10 0x20 'x'

will successively push the values 0x10, 0x20, and the character 'x' onto the
data stack.  The character 'x' will be at the top of the data stack after these
3 instructions.

Numeric values can be represented in binary, octal, decimal, and hex.  Binary
values start with the two characters "0b" followed by a sequence of binary
digits; octal numbers start with a "0" followed by a sequence of octal digits;
decimal values can start with a "+" or "-" have a non-zero first digit and have
zero or more decimal digits; and hex values start with the two characters "0X"
followed by a sequence of hex digits.

Examples of equivalent numeric values are:
  binary:   0b01  0b10010
  octal:    01    022
  decimal:  1     18
  hex:      0x1   0x12

See the COMPUTED VALUES section for using computed values in the assembler.

There are four ways to specify strings in the assembler.  Simply stating the
string

  "Hello World!"

puts the characters in the string onto the data stack with the letter 'H' at the
top of the data stack.  I.e., the individual push operations are

  '!' 'd' 'l' ... 'e' 'H'

Prepending a 'N' before the double quote, like

  N"Hello World!"

puts a null-terminated string onto the data stack.  I.e., the value under the
'!' will be a 0x00 and the instruction sequence would be

  0x0 '!' 'd' 'l' ... 'e' 'H'

Forth uses counted strings, which are specified here as

  C"Hello World!"

In this case the number of characters, 12, in the string is pushed onto the data
stack after the 'H', i.e., the instruction sequence would be

  '!' 'd' 'l' ... 'e' 'H' 12

Finally, a lesser-counted string specified like

  c"Hello World!"

is similar to the Forth-like counted string except that the value pushed onto
the data stack is one less than the number of characters in the string.  Here
the value pushed onto the data stack after the 'H' would be 11 instead of 12.

Simple strings are useful for constructing more complex strings in conjunction
with other string functions.   For example, to transmit the hex values of the
top 2 values in the data stack, do something like:

  ; move the top 2 values to the return stack
  >r >r
  ; push the tail of the message onto the data stack
  N"\n\r"
  ; convert the 2 values to 2-digit hex values, LSB deepest in the stack
  r> .call(string_byte_to_hex)
  r> .call(string_byte_to_hex)
  ; pre-pend the identification message
  "Message:  "
  ; transmit the string, using the null terminator to terminate the loop
  :loop_transmit .outport(O_UART_TX) .jumpc(loop_transmit,nop) drop

A lesser-counted string would be used like:

  c"Status Message\r\n"
  :loop_msg swap .outport(O_UART_TX) .jumpc(loop_msg,1-) drop

These four string formats can also be used for variable definitions.  For
example 3 variables could be allocated and initialized as follows:

  .memory ROM myrom
  .variable fred N"fred"
  .variable joe  c"joe"
  .variable moe  "moe"

These are equivalent to

  .variable fred 'f' 'r' 'e' 'd'  0
  .variable joe   2  'j' 'o' 'e'
  .variable moe  'm' 'o' 'e'

with 5 bytes allocated for the variable fred, 4 bytes for joe, and 3 bytes for
moe.

The following escaped characters are recognized:

  '\0'     null character
  '\a'     bell
  '\b'     backspace
  '\f'     form feed
  '\n'     line feed
  '\r'     carriage return
  '\t'     horizontal tab
  "\0ooo"  3-digit octal value
  "\xXX"   2-digit hex value where X is one of 0-9, a-f, or A-F
  "\Xxx"   alternate form for 2-digit hex value
  "\\"     backslash character

Unrecognized escaped characters are simple treated as that character.  For
example, '\m' is treated as the single character 'm' and '\'' is treated as the
single quote character.


INSTRUCTIONS
================================================================================

The 43 instructions are as follows (see core/9x8/doc/opcodes.html for detailed
descriptions).  Here, T is the top of the data stack, N is the next-to-top of
the data stack, and R is the top of the return stack.  All of these are the
values at the start of the instruction.

The nop instruction does nothing:

  nop           no operation

Mathematical operations drop one value from the data stack and replace the new
top with the state value:

  &             bitwise and of N and T
  +             N + T
  -             N - T
  ^             bitwise exclusive or of N and T
  or            bitwise or of N and T

Push the carry bit for addition or subtraction onto the data stack (see
lib/9x8/math.s for examples on using +c and -c for multi-byte arithmetic):

  +c            carry bit for N+T
  -c            carry bit for N-T

Increment and decrement replace the top of the data stack with the stated
result.

  1+            replace T with T+1
  1-            replace T with T-1

Comparison operations replace the top of the data stack with the results of the
comparison:

  -1<>          replace T with -1 if T != -1, otherwise set T to 0
  -1=           replace T with 0 if T != -1, otherwise leave T as -1
  0<>           replace T with -1 if T != 0, otherwise leave T as 0
  0=            replace T with -1 if T == 0, otherwise set T to 0

Shift/rotate operations replace the top of the data with with the result of the
specified shift/rotate.

  0>>           shift T right one bit and set the msb to 0
  1>>           shift T right 1 bit and set the msb to 1
  <<0           shift T left 1 bit and set the lsb to 0
  <<1           shift T left 1 bit and set the lsb to 1
  <>         rotate T right 1 bit
  msb>>         shift T right 1 bit and set the msb to the old msb

Note:  There is no "<r            pushd T onto the return stack and drop T from the data stack
  drop          drop T from the data stack
  dup           push T onto the data stack
  nip           drop N from the data stack
  over          push N onto the data stack
  push          push a single byte onto the data stack, see the preceding DATA
                and STRINGS section
  r>            push R onto the data stack and drop R from the return stack
  r@            push R onto the data stack
  swap          swap N and T

Jump and call and their conditional variants are as follows and must use the
associated macro:

  call          call instruction -- use the .call macro
  callc         conditional call instruction -- use the .callc macro
  jump          jump instruction -- use the .jump macro
  jumpc         conditional jump instruction -- use the .jumpc macro
  return        return instruction -- use the .return macro

See the MEMORY section for details for these memory operations.  T is the
address for the instructions, N is the value stored.  Chained fetches insert the
value below T.  Chained stores drop N.

  fetch         memory fetch, replace T with the value fetched
  fetch+        chained memory fetch, retain and increment the address
  fetch-        chained memory fetch, retain and decrement the address
  store         memory store, drop T (N is the next value of T)
  store+        chained memory store, retain and increment the address
  store-        chained memory store, retain and decrement the address

See the INPORT and OUTPORT section for details for the input and output port
operations:

  inport        input port operation
  outport       output port operation

The .call, .callc, .jump, and .jumpc macros encode the 3 instructions required
to perform a call or jump along with the subsequent instructions.  The default
third instructions is "nop" for .call and .jump and it is "drop" for .callc and
.jumpc.  The default can be changed by specifying the optional second argument.
The .call and .callc macros must specify a function identified by the .function
directive and the .jump and .jumpc macros must specify a label.

The .function directive takes the name of the function and the function body.
Function bodies must end with a .return or a .jump macro.  The .main directive
defines the body of the main function, i.e., the function at which the processor
starts.

The .include directive is used to read additional assembly code.  You can, for
example, put the main function in uc.s, define constants and such in consts.s,
define the memories and variables in ram.s, and include UART utilities in
uart.s.  These files could be included in uc.s through the following lines:

  .include consts.s
  .include myram.s
  .include uart.s

The assembler only includes functions that can be reached from the main
function.  Unused functions will not consume instruction space.


INPORT and OUTPORT
================================================================================

The INPORT and OUTPORT configuration commands are used to specify 2-state inputs
and outputs.  For example

  INPORT 8-bit i_value I_VALUE

specifies a single 8-bit input signal named "i_value" for the module.  The port
is accessed in assembly by ".inport(I_VALUE)" which is equivalent to the
two-instruction sequence "I_VALUE inport".  To input an 8-bit value from a FIFO
and send a single-clock-cycle wide acknowledgment strobe, use

  INPORT 8-bit,strobe i_fifo,o_fifo_ack I_FIFO

The assembly ".inport(I_FIFO)" will automatically send an acknowledgment strobe
to the FIFO through "o_fifo_ack".

A write port to an 8-bit FIFO is similarly specified by

  OUTPORT 8-bit,strobe o_fifo,o_fifo_wr O_FIFO

The assembly ".outport(O_FIFO)" which is equivalent to "O_FIFO outport drop"
will automatically send a write strobe to the FIFO through "o_fifo_wr".

Multiple signals can be packed into a single input or output port by defining
them in comma separated lists.  The associated bit masks can be defined
coincident with the port definition as follows:

  INPUT 1-bit,1-bit i_fifo_full,i_fifo_empty I_FIFO_STATUS
  CONSTANT C_FIFO_STATUS__FULL  0x02
  CONSTANT C_FIFO_STATUS__EMPTY 0x01

Checking the "full" status of the FIFO can be done by the following assembly
sequence:

  .inport(I_FIFO_STATUS) C_FIFO_STATUS__FULL &

Multiple bits can be masked using a computed value as follows (see below for
more details):

  .inport(I_FIFO_STATUS) ${C_FIFO_STATUS__FULL|C_FIFO_STATUS__EMPTY} &

The "${...}" creates an instruction to push the 8-bit value in the braces onto
the data stack.  The computation is performed using the Python "eval" function
in the context of the program constants, memory addresses, and memory sizes.

Preceding all of these by

  PORTCOMMENT external FIFO

produces the following in the Verilog module statement.  The I/O ports are
listed in the order in which they are declared.

  // external FIFO
  input  wire       [7:0] i_fifo,
  output reg              o_fifo_ack,
  output reg        [7:0] o_fifo,
  output reg              o_fifo_wr,
  input  wire             i_fifo_full,
  input  wire             i_fifo_empty

The HDL to implement the inputs and outputs is computer generated.  Identifying
the port name in the architecture file eliminates the possibility of
inconsistent port numbers between the HDL and the assembly.  Specifying the bit
mapping for the assembly code immediately after the port definition helps
prevent inconsistencies between the port definition and the bit mapping in the
assembly code.

The normal initial value for an outport is zero.  This can be changed by
including an optional initial value as follows.  This initial value will be
applied on system startup and when the micro controller is reset.

  OUTPORT 4-bit=4'hA o_signal O_SIGNAL

An isolated output strobe can also be created using:

  OUTPORT strobe o_strobe O_STROBE

The assembly ".outstrobe(O_STROBE)" which is equivalent to "O_STROBE outport"
is used to generate the strobe.  Since "O_STROBE" is a strobe-only outport, the
".outport" macro cannot be used with it.  Similarly, attempting to use the
".outstrobe" macro will generate an error if it is invoked with an outport
that does have data.

A single-bit "set-reset" input port type is also included.  This sets a register
when an external strobe is received and clears the register when the port is
read.  For example, to capture an external timer for a polled-loop, include the
following in the architecture file:

  PORTCOMMENT external timer
  INPORT set-reset i_timer I_TIMER

The following is the assembly code to conditionally call two functions when the
timer event is encountered:

  .inport(I_TIMER)
    .callc(timer_event_1,nop)
    .callc(timer_event_2)

The "nop" in the first conditional call prevents the conditional from being
dropped from the data stack so that it can be used by the subsequent conditional
function call.

The input from a set-reset INPORT is a pure flag.  I.e., either all of the bits
are zero or all of the bits are one.  This can be used as part of executing a
loop a fixed number of times.  For example, the inperiod argument of the
servo_motor peripheral can be used to receive a strobe every time the PWM goes
high.  The following loop will wait for 10 occurrences of the rising edge of the
servo_motor PWM before proceeding to the next block of code:

  10 :loop .inport(I_INPERIOD) + .jumpc(loop,nop) drop


PERIPHERAL
================================================================================

Peripherals are implemented via Python modules.  For example, an open drain I/O
signal, such as is required for an I2C bus, does not fit the INPORT and OUTPORT
functionality.  Instead, an "open_drain" peripheral is provided by the Python
script in "core/9x8/peripherals/open_drain.py".  This puts a tri-state I/O in
the module statement, allows it to be read through an "inport" instruction, and
allows it to be set low or released through an "outport" instruction.  An I2C
bus with separate SCL and SDA ports can then be incorporated into the processor
as follows:

  PORTCOMMENT     I2C bus
  PERIPHERAL      open_drain      inport=I_SCL \
                                  outport=O_SCL \
                                  iosignal=io_scl
  PERIPHERAL      open_drain      inport=I_SDA \
                                  outport=O_SDA \
                                  iosignal=io_sda

The default width for this peripheral is 1 bit.  The module statement will then
include the lines

  // I2C bus
  inout  wire     io_scl,
  inout  wire     io_sda

The assembly code to set the io_scl signal low is "0 .outport(O_SCL)" and to
release it is "1 .outport(O_SCL)".  These instruction sequences are actually
"0 O_SCL outport drop" and "1 O_SCL outport drop" respectively.  The "outport"
instruction drops the top of the data stack (which contained the port number)
and sends the next-to-the-top of the data stack to the designated output port.

Two examples of I2C device operation are included in the examples directory.

The following peripherals are provided:
  adder_16bit   16-bit adder/subtractor
  AXI4_Lite_Master
                32-bit read/write AXI4-Lite Master
                Note:  The synchronous version has been tested on hardware.
  AXI4_Lite_Slave_DualPortRAM
                dual-port-RAM interface for the micro controller to act as an
                AXI4-Lite slave
  AXI_Stream_Slave
                AXI-Stream slave with optional TLast signal
  big_inport    shift reads from a single INPORT to construct a wide input
  big_outport   shift writes to a single OUTPORT to construct a wide output
  counter       counter for number of received high cycles from signal
  inFIFO_async  input FIFO with an asynchronous write clock
  latch         latch wide inputs for sampling
  monitor_stack simulation diagnostic (see below)
  open_drain    for software-implemented I2C buses or similar
  outFIFO_async output FIFO with an asynchronous read clock
  PWM_8bit      PWM generator with an 8-bit control
  servo_motor   PWM modulation suitable for servo motor or similar control
  stepper_motor stepper motor controller with acceleration
  timer         timing for polled loops or similar
  trace         simulation diagnostic (see below)
  UART          bidirectional UART
  UART_Rx       receive UART
  UART_Tx       transmit UART
  wide_strobe   1 to 8 bit strobe generator

The following command illustrates how to display the help message for
peripherals:

  echo "ARCHITECTURE core/9x8 Verilog" | ssbcc -P "big_inport help" - | less

User defined peripherals can be in the same directory as the architecture file
or a subdirectory named "peripherals".


INTERRUPTS
================================================================================

Interrupts are enabled by including an interrupt peripheral in the architecture
file and an interrupt handler in the assembly code.

Incorporating an interrupt adds the following output ports as strobes:

  O_INTERRUPT_DIS       disable interrupts
  O_INTERRUPT_ENA       enable interrupts

and the following 3 macros

  .dis          disable interrupts
                Note:  This is equivalent to .outstrobe(O_INTERRUPT_DIS)
  .ena          enable interrupts
                Note:  This is equivalent to .outstrobe(O_INTERRUPT_ENA)
  .returni      return from interrupt handler and enable interrupts
                Note:  This is equivalent to the 3 instruction sequence
                       "O_INTERRUPT_ENA return outport"
                Note:  The assembler prohibits the ".return" macro in the
                       interrupt handler and the ".returni" macro anywhere other
                       than the interrupt handler.

See examples/interrupt for a demonstration of an interrupt handler for two
events, one external to the processor and one internal to the processor.

The interrupt handler included in core/9x8/peripherals handles one to eight
interrupt sources.  See core/9x8/doc/interrupt.html for instructions on creating
custom interrupt peripherals, possibly with more interrupt sources.


PARAMETER and LOCALPARAM
================================================================================

Parameters are incorporated through the PARAMETER and LOCALPARAM configuration
commands.  For example, the clock frequency in hertz is needed for UARTs for
their baud rate generator.  The configuration command

  PARAMETER G_CLK_FREQ_HZ 97_000_000

specifies the clock frequency as 97 MHz.  The HDL instantiating the processor
can change this specification.  The frequency can also be changed through the
command-line invocation of the computer compiler.  For example,

  ssbcc -G "G_CLK_FREQ_HZ=100_000_000" myprogram.9x8

specifies that a frequency of 100 MHz be used instead of the default frequency
of 97 MHz.

The LOCALPARAM configuration command can be used to specify parameters that
should not be changed by the surrounding HDL.  For example,

  LOCALPARAM L_VERSION 24'h00_00_00

specifies a 24-bit parameter named "L_VERSION".  The 8-bit major, minor, and
build sections of the parameter can be accessed in an assembly program using
"L_VERSION[16+:8]", "L_VERSION[8+:8]", and "L_VERSION[0+:8]".

For both parameters and localparams, the default range is "[0+:8]".  The
instruction memory is initialized using the parameter value during synthesis,
not the value used to initialize the parameter.  That is, the instruction memory
initialization will be:

  s_opcodeMemory[...] = { 1'b1, L_VERSION[16+:8] };

The value of the localparam can be set when the computer compiler is run using
the "-G" option.  For example,

  ssbcc -G "L_VERSION=24'h01_04_03" myprogram.9x8

can be used in a makefile to set the version number for a release without
modifying the micro controller architecture file.


DIAGNOSTICS AND DEBUGGING
================================================================================

A 3-character, human readable version of the opcode can be included in
simulation waveform outputs by adding "--display-opcode" to the ssbcc command.

The stack health can be monitored during simulation by including the
"monitor_stack" peripheral through the command line.  For example, the LED
flasher example can be generated using

  ssbcc -P monitor_stack led.9x8

This allows the architecture file to be unchanged between simulation and an FPGA
build.

Stack errors include underflow and overflow, malformed data validity, and
incorrect use of the values on the return stack (returns to data values and data
operations on return addresses).  Other errors include out-of-range for memory,
inport, and outport operations.

When stack errors are detected the last 50 instructions are dumped to the
console and the simulation terminates.  The dump includes the PC, numeric
opcode, textual representation of the opcode, data stack pointer, next-to-top of
the data stack, top of the data stack, top of the return stack, and the return
stack pointer.  Invalid stack values are displayed as "XX".  The length of the
history dumped is configurable.

Out-of-range PC checks are also performed if the instruction space is not a
power of 2.

A "trace" peripheral is also provided that dumps the entire execution history.
This was used to validate the processor core.


MEMORY ARCHITECTURE
================================================================================

The DATA_STACK, RETURN_STACK, INSTRUCTION, and MEMORY configuration commands
allocate memory for the data stack, return stack, instruction ROM, and memory
RAM and ROM respectively.  The data stack, return stack, and memories are
normally instantiated as dual-port LUT-based memories with asynchronous reads
while the instruction memory is always instantiated with a synchronous read
architecture.

The COMBINE configuration command is used to coalesce memories and to convert
LUT-based memories to synchronous SRAM-based memories.  For example, the large
SRAMs in modern FPGAs are ideal for storing the instruction opcodes and their
dual-ported access allows either the data stack or the return stack to be
stored in a relatively small region at the end of the large instruction memory.
Memories, which required dual-ported operation, can also be instantiated in
large RAMs either individually or in combination with each other.  Conversion
to SRAM-based memories is also useful for FPGA architectures that do not have
efficient LUT-based memories.

The INSTRUCTION configuration allocates memory for the processor instruction
space.  It has the form "INSTRUCTION N" or "INSTRUCTION N*M" where N must be a
power of 2.  The first form is used if the desired instruction memory size is a
power of 2.  The second form is used to allocate M memory blocks of size N
where M is not a power of 2.  For example, on an Altera Cyclone III, the
configuration command "INSTRUCTION 1024*3" allocates three M9Ks for the
instruction space, saving one M9K as compared to the configuration command
"INSTRUCTION 4096".

The DATA_STACK configuration command allocates memory for the data stack.  It
has the form "DATA_STACK N" where N is the commanded size of the data stack.
N must be a power of 2.

The RETURN_STACK configuration command allocates memory for the return stack and
has the same format as the DATA_STACK configuration command.

The MEMORY configuration command is used to define one to four memories, either
RAM or ROM, with up to 256 bytes each.  If no MEMORY configuration command is
issued, then no memories are allocated for the processor.  The MEMORY
configuration command has the format "MEMORY {RAM|ROM} name N" where
"{RAM|ROM}" specifies either a RAM or a ROM, name is the name of the memory and
must start with an alphabetic character, and the size of the memory, N, must be
a power of 2.  For example, "MEMORY RAM myram 64" allocates 64 bytes of memory
to form a RAM named myram.  Similarly, "MEMORY ROM lut 256" defines a 256 byte
ROM named lut.  More details on using memories is provided in the next section.

The COMBINE configuration command can be used to combine the various memories
for more efficient processor implementation as follows:

  COMBINE INSTRUCTION,
  COMBINE 
  COMBINE ,
  COMBINE 

where  is one of DATA_STACK, RETURN_STACK, or a list of one
or more ROMs and  is a list of one or more RAMs and/or ROMs.  The first
configuration command reserves space at the end of the instruction memory for
the DATA_STACK, RETURN_STACK, or listed ROMs.

The SRAM_WIDTH configuration command is used to make the memory allocations more
efficient when the SRAM block width is more than 9 bits.  For example,
Altera's Cyclone V family has 10-bit wide memory blocks and the configuration
command "SRAM_WIDTH 10" is appropriate.  The configuration command
sequence

  INSTRUCTION     1024
  RETURN_STACK    32
  SRAM_WIDTH      10
  COMBINE         INSTRUCTION,RETURN_STACK

will use a single 10-bit memory entry for each element of the return stack
instead of packing the 10-bit values into two memory entries of a 9-bit wide
memory.

The following illustrates a possible configuration for a Spartan-6 with a
2048-long SRAM and relatively large 64-deep data stack.  The data stack will be
in the last 64 elements of the instruction memory and the instruction space will
be reduced to 1984 words.

  INSTRUCTION   2048
  DATA_STACK    64
  COMBINE       INSTRUCTION,DATA_STACK

The following illustrates a possible configuration for a Cyclone-III with three
M9Ks for the instruction ROM and the data stack.

  INSTRUCTION   1024*3
  DATA_STACK    64
  COMBINE       INSTRUCTION,DATA_STACK

WARNING:  Some devices, such as Xilinx' Spartan-3A devices, do not support
asynchronous reads, so the COMBINE configuration command does not work for them.

WARNING:  Xilinx XST does not correctly infer a Block RAM when the
"COMBINE INSTRUCTION,RETURN_STACK" configuration command is used and the
instruction space is 1024 instructions or larger.  Xilinx is supposed to fix
this in a future release of Vivado so the fix will only apply to 7-series or
later FPGAs.


MEMORY
================================================================================

The MEMORY configuration command is used as follows to allocate a 128-byte RAM
named "myram" and to allocate a 32-byte ROM named "myrom".  Zero to four
memories can be allocated, each with up to 256 bytes.

  MEMORY RAM myram 128
  MEMORY ROM myrom  32

The assembly code to lay out the memory uses the ".memory" directive to identify
the memory and the ".variable" directive to identify the symbol and its content.
Single or multiple values can be listed and "*N" can be used to identify a
repeat count.

  .memory RAM myram
  .variable a 0
  .variable b 0
  .variable c 0 0 0 0
  .variable d 0*4

  .memory ROM myrom
  .variable coeff_table 0x04
                        0x08
                        0x10
                        0x20
  .variable hello_world N"Hello World!\r\n"

Single values are fetched from or stored to memory using the following assembly:

  .fetchvalue(a)
  0x12 .storevalue(b)

Multi-byte values are fetched or stored as follows.  This copies the four values
from coeff_table, which is stored in a ROM, to d.

  .fetchvector(coeff_table,4) .storevector(d,4)

The memory size is available using computed values (see below) and can be used
to clear the entire memory, etc.

The available single-cycle memory operation macros are:
  .fetch(mem_name)      replaces T with the value at the address T in the memory
                        mem_name
                        Note:  .fetchram(var_name) is safer.
  .fetch+(mem_name)     pushes the value at address T in the memory mem_name
                        into the data stack below T and increments T
                        Note:  This is useful for fetching successive values
                               from memory into the data stack.
                        Note:  .fetchram+(var_name) is safer.
  .fetch-(mem_name)     similar to .fetch+ but decrements T
                        Note:  .fetchram-(var_name) is safer.
  .store(ram_name)      stores N at address T in the RAM ram_name, also drops
                        the top of the data stack
                        Note:  .storeram(var_name) is safer.
  .store+(ram_name)     stores N at address T in the RAM ram_name, also drops N
                        from the data stack and increments T
                        Note:  .storeram+(var_name) is safer.
  .store-(ram_name)     similar to .store+ but decrements T
                        Note:  .storeram-(var_name) is safer.

The following multi-cycle macros provide more generalized access to the
memories:
  .fetchindexed(var_name)
                        uses the top of the data stack as an index into var_name
                        Note:  This is equivalent to the 3 instruction sequence
                               "var_name + .fetch(mem_name)"
  .fetchoffset(var_name,offset)
                        fetches the single-byte value of var_name offset by
                        "offset" bytes
                        Note:  This is equivalent to
                                 "${var_name+offset} .fetch(mem_name)"
                               where mem_name is the memory in which var_name is
                               stored.
  .fetchram(var_name)   is similar to the .fetch(mem_name) macro except that the
                        variable name is used to identify the memory instead of
                        the name of the memory
  .fetchram+(var_name)  is similar to the .fetch+(mem_name) macro except that
                        the variable name is used to identify the memory instead
                        of the name of the memory
  .fetchram-(var_name)  is similar to the .fetch-(mem_name) macro except that
                        the variable name is used to identify the memory instead
                        of the name of the memory
  .fetchvalue(var_name) fetches the single-byte value of var_name
                        Note:  This is equivalent to
                                 "var_name .fetch(mem_name)"
                               where mem_name is the memory in which var_name is
                               stored.
  .fetchvector(var_name,N)
                        fetches N values starting at var_name into the data
                        stack with the value at var_name at the top and the
                        value at var_name+N-1 deep in the stack.
                        Note:  This is equivalent N+1 operation sequence
                                 "${var_name+N-1} .fetch-(mem_name) ...
                                 .fetch-(mem_name) .fetch(mem_name)"
                               where ".fetch-(mem_name)" is repeated N-1 times.
  .storeindexed(var_name)
                        uses the top of the data stack as an index into var_name
                        into which to store the next-to-top of the data stack.
                        Note:  This is equivalent to the 4 instruction sequence
                                 "var_name + .store(mem_name) drop".
                        Note:  The default "drop" instruction can be overriden
                               by providing the optional second argument
                               similarly to the .storevalue macro.
  .storeoffset(var_name,offset)
                        stores the single-byte value at the top of the data
                        stack at var_name offset by "offset" bytes
                        Note:  This is equivalent to
                                 "${var_name+offset} .store(mem_name) drop"
                               where mem_name is the memory in which var_name is
                               stored.
                        Note:  The default "drop" instruction can be replaced by
                               providing the optional third argument.
  .storeram(var_name)   is similar to the .store(mem_name) macro except that the
                        variable name is used to identify the RAM instead of the
                        name of the RAM
  .storeram+(var_name)  is similar to the .store+(mem_name) macro except that
                        the variable name is used to identify the RAM instead of
                        the name of the RAM
  .storeram-(var_name)  is similar to the .store-(mem_name) macro except that
                        the variable name is used to identify the RAM instead of
                        the name of the RAM
  .storevalue(var_name) stores the single-byte value at the top of the data
                        stack at var_name
                        Note:  This is equivalent to
                                 "var_name .store(mem_name) drop"
                        Note:  The default "drop" instruction can be replaced by
                               providing the optional second argument.  For
                               example, the following instruction will store and
                               then decrement the value at the top of the data
                               stack:
                                 .storevalue(var_name,1-)
  .storevector(var_name,N)
                        Does the reverse of the .fetchvector macro.
                        Note:  This is equivalent to the N+2 operation sequence
                                 "var_name
                                 .store+(mem_name) ...  .store+(mem_name)
                                 .store(mem_name) drop"
                               where ".store+(mem_name)" is repeated N-1 times.

The .fetchvector and .storevector macros are intended to work with values stored
MSB first in memory and with the MSB toward the top of the data stack,
similarly to the Forth language with multi-word values.  To demonstrate how
this data structure works, consider the examples of decrementing and
incrementing a two-byte value on the data stack:

  ; Decrement a 2-byte value
  ;   swap 1- swap      - decrement the LSB
  ;   over -1=          - puts -1 on the top of the data stack if the LSB rolled
  ;                       over from 0 to -1, puts 0 on the top otherwise
  ;   +                 - decrements the MSB if the LSB rolled over
  ; ( u_LSB u_MSB - u_LSB' u_MSB' )
  .function decrement_2byte
  swap 1- swap over -1= .return(+)

  ; Increment a 2-byte value
  ;   swap 1+ swap      - increment the LSB
  ;   over 0=           - puts -1 on the top of the data stack if the LSB rolled
  ;                       over from 0xFF to 0, puts 0 on the top otherwise
  ;   -                 - increments the MSB if the LSB rolled over (by
  ;                       subtracting -1)
  ; ( u_LSB u_MSB - u_LSB' u_MSB' )
  .function increment_2byte
  swap 1+ swap over 0= .return(-)


COMPUTED VALUES
================================================================================

Computed values can be pushed on the stack using a "${...}" where the "..." is
evaluated in Python and cannot have any spaces.

For example, a loop that should be run 5 times can be coded as:

  ${5-1} :loop ... .jumpc(loop,1-) drop

which is a clearer indication that the loop is to be run 5 times than is the
instruction sequence

  4 :loop ...

Constants can be accessed in the computation.  For example, a block of memory
can be allocated as follows:

  .constant C_RESERVE
  .memory RAM myram
  ...
  .variable reserved 0*${C_RESERVE}

and the block of reserved memory can be cleared using the following loop:

  ${C_RESERVE-1} :loop 0 over .storeindexed(reserved) .jumpc(loop,1-) drop

The offsets of variables in their memory can also be accessed through a computed
value.  The value of reserved could also be cleared as follows:

  ${reserved-1} ${C_RESERVE-1} :loop >r
    0 swap .store-(myram)
  r> .jumpc(loop,-1) drop drop

This body of this version of the loop is the same length as the first version.
In general, it is better to use the memory macros to access variables as they
ensure the correct memory is accessed.

The sizes of memories can also be accessed using computed values.  If "myram" is
a RAM, then "${size['myram']}" will push the size of "myram" on the stack.  As
an example, the following code will clear the entire RAM:

  ${size['myram']-1} :loop 0 swap .jumpc(loop,.store-(myram)) drop

The lengths of I/O signals can also be accessed using computed values.  If
"o_mask" is a mask, then "${size['o_mask']}" will push the size of the mask on
the stack and "${2**size['o_mask']-1}" will push a value that sets all the bits
of the mask.  The I/O signals include I/O signals instantiated by peripherals.
For example, for the configuration command

  PERIPHERAL big_outport outport=O_BIG outsignal=o_big width=47

the width of the output signal is accessible using "${size['o_big']}".  You can
set the wide signal to all zeroes using:

  ${(size['o_big']+7)/8-1} :loop 0 .outport(O_BIG) .jumpc(loop,1-) drop

The sizes of variables, i.e., how many bytes of memory they occupy, are also
provided.


MACROS
================================================================================

There are 3 types of macros used by the assembler.

The first kind of macros are built in to the assembler and are required to
encode instructions that have embedded values or have mandatory subsequent
instructions.  These include function calls, jump instructions, function return,
and memory accesses as follows:
  .call(function,[op])
  .callc(function,[op])
  .fetch(ramName)
  .fetch+(ramName)
  .fetch-(ramName)
  .jump(label,[op])
  .jumpc(label,[op])
  .return([op])
  .store(ramName)
  .store+(ramName)
  .store-(ramName)

The second kind of macros are designed to ease access to input and output
operations and for memory accesses and to help ensure these operations are
correctly constructed.  These are defined as python scripts in the
core/9x8/macros directory and are automatically loaded into the assembler.
These macros are:
  .fetchindexed(variable)
  .fetchoffset(variable,ix)
  .fetchvalue(variableName)
  .fetchvector(variableName,N)
  .inport(I_name)
  .outport(O_name[,op])
  .outstrobe(O_name)
  .storeindexed(variableName[,op])
  .storeoffset(variableName,ix[,op])
  .storevalue(variableName[,op])
  .storevector(variableName,N)

The third kind of macro is user-defined macros.  These macros must be registered
with the assembler using the ".macro" directive.

For example, the ".push32" macro is defined by macros/9x8/push32.py and can be
used to push 32-bit (4-byte) values onto the data stack as follows:

  .macro push32
  .constant C_X 0x87654321
  .main
    ...
    .push32(0x12345678)
    .push32(C_X)
    .push32(${0x12345678^C_X})
    ...

The following macros are provided in macros/9x8:
  .push16(v)    push the 16-bit (2-byte) value "v" onto the data stack with the
                MSB at the top of the data stack
  .push24(v)    push the 24-bit (3-byte) value "v" onto the data stack with the
                MSB at the top of the data stack
  .push32(v)    push the 32-bit (4-byte) value "v" onto the data stack with the
                MSB at the top of the data stack
  .pushByte(v,ix)
                push the ix'th byte of v onto the data stack
                Note:  ix=0 designates the LSB

Directories are searched in the following order for macros:
  .
  ./macros
  include paths specified by the '-M' command line option.
  macros/9x8

The python scripts in core/9x8/macros and macros/9x8 can be used as design
examples for user-defined macros.  The assembler does some type checking based
on the list provided when the macro is registered by the "AddMacro" method, but
additional type checking is often warranted by the macro "emitFunction" which
emits the actual assembly code.  The ".fetchvector" and ".storevector" macros
demonstrates how to design variable-length macros.  Several macros in
core/9x8/macros illustrate designing macros with optional arguments.

It is not an error to repeat the ".macro MACRO_NAME" directive for user-defined
macros.  The assembler will issue a fatal error if a user-defined macro
conflicts with a built-in macro.


CONDITIONAL COMPILATION
================================================================================

Conditional compilation is accepted in the architecture file and in the assembly
source files and is based on whether or not the referenced symbol is defined.

In the architecture file, symbols are defined by CONSTANT, INPORT, OUTPORT, and
PERIPHERAL statements, or by the "-D D_name" argument to ssbcc.  The line

  .IFDEF name
  
  .ENDIF

will then include the lines "" if the symbol "name" is defined.
The ".IFNDEF" command is similar except that the statements included if the
symbol is not defined.  A ".ELSE" is also provided.

In an assembly file directives are conditionally included using the .IFDEF and
.IFNDEF directives, an optional .ELSE directive, and the terminating .ENDIF
directive.  Note that this is done at the directive level, i.e. function
declarations, memory declarations, and so forth.  Within a function code is
conditionally included starting with a ".ifdef(name)" or a ".ifndef(name)", an
optional ".else" and a terminating ".endif".

For example, a diagnostic UART can be conditionally included in a program by
including the following lines in the architecture file:

  .IFDEF D_ENABLE_UART
  PORTCOMMENT Diagnostic UART
  PERIPHERAL UART_Tx outport=O_UART_TX ...
  .ENDIF

A "uart_tx" function can be optionally created using code similar to the
following.  Note that the symbol for the .outport macro is used to determine
whether or not the function is defined since that is more closely related to
whether or not the function can be defined.  The function definition must
preceed any ".ifdef(uart_tx)" conditionals used to output diagnostics.

  .IFDEF O_UART_TX
  .function uart_tx
    :loop .outport(O_UART_TX) .jumpc(loop,nop) .return(drop)
  .ENDIF

Diagnostics in the assembly code can be included using either

  .ifdef(D_ENABLE_UART) N"Msg\r\n" .call(uart_tx) .endif

or

  .ifdef(uart_tx) N"Msg\r\n" .call(uart_tx) .endif

Conditional compilation cannot cross file boundaries.

The assembler also recognizes the ".define" directive.  For example, specific
diagnostics could be enabled if the UART is instantiated as follows:

  .IFDEF O_UART_TX
  .define D_DEBUG_FUNCTION_A
  .ENDIF

  ...

  .ifdef(D_DEBUG_FUNCTION_A) N"Debug Msg\r\n" .call(uart_tx) .endif

The following code illustrates how to preclude multiple attempted inclusions of
an assembly library file.

  ; put these two lines near the top of the file
  .IFNDEF D_FILENAME_INCLUDED
  .define D_FILENAME_INCLUDED
  ; put the library body here
  ...
  ; put this line at the bottom of the file
  .ENDIF ; .IFNDEF D_FILENAME_INCLUDED

The ".INCLUDE" configuration command can be used to read configuration commands
from additional sources.  For example, the following code will conditionally
include a general UART library if the outport O_UART_TX is defined:

  .IFDEF O_UART_TX
  .INCLUDE uart.s
  .ENDIF


SIMULATIONS
================================================================================

Simulations have been performed with Icarus Verilog, Verilator, and Xilinx'
ISIM.  Icarus Verilog is good for short, simple simulations and is used for the
core and peripheral test benches; Verilator for long simulations of large,
complex systems; and ISIM when Xilinx-specific cores are used.  Verilator is
the fastest simulators I've encountered.  Verilator is also used for lint
checking in the core test benches.

The test benches for the core and the peripherals require Icarus Verilog and
Verilator.


MEM INITIALIZATION FILE
================================================================================

A memory initialization file is produced during compilation.  This file can be
used with tools such as Xilinx' data2mem to modify the SRAM contents without
having to rebuild the entire system.  It is restricted to the opcode memory
initialization.  The file must be processed before it can be used by specific
tools, see doc/MemoryInitialization.html.

WARNING:  The values of parameters used in the assembly code must match the
instantiated design.


THEORY OF OPERATION
================================================================================

Registers are used for the top of data stack, "T", and the next-to-top of the
data stack, "N".  The data stack is a separate memory.  This means that the
"DATA_STACK N" configuration command actually allows N+2 values in the data
stack since T and N are not stored in the N-element deep data stack.

The return stack is similar in that "R" is the top of the return stack and the
"RETURN_STACK N" allocates an additional N words of memory.  The return stack is
the wider of the 8-bit data width and the program counter width.

The program counter is always either incremented by 1 or is set to an address
as controlled by jump, jumpc, call, callc, and return instructions.  The
registered program counter is used to read the next opcode from the instruction
memory and this opcode is also registered in the memory.  This means that there
is a 1 clock cycle delay between the address changing and the associated
instruction being performed.  This is also part of the architecture required to
have the processor operate at one instruction per clock cycle.

Separate ALUs are used for the program counter, adders, logical operations, etc.
and MUXes are used to select the values desired for the destination registers.
The instruction execution consists of translating the upper 6 msb of the opcode
into MUX settings and performing opcode-dependent ALU operations as controlled
by the 3 lsb of the opcode (during the first half of the clock cycle) and then
setting the T, N, R, memories, etc. as controlled by the computed MUX settings.

The "core.v" file is the code for these operations.  Within this file there are
several "@xxx@" strings that specify where the computer compiler is to insert
code such as I/O declarations, memories, inport interpretation, outport
generation, peripherals, etc.

The file structure, i.e., putting the core and the assembler in "core/9x8"
should facilitate application-specific modification of processor.  For example,
the store+, store-, fetch+, and fetch- instructions could be replaced with
additional stack manipulation operations, arithmetic operations with 2 byte
results, etc.  Simply copy the "9x8" directory to something like "9x8_XXX" and
make your modifications in that directory.  The 8-bit peripherals should still
work, but the 9x8 library functions may need rework to accommodate the
modifications.


MISCELLANEOUS
================================================================================

Features and peripherals are still being added and the documentation is
incomplete.  The output HDL is currently restricted to Verilog although a VHDL
package file is automatically generated by the computer compiler.

The "INVERT_RESET" configuration command is used to indicate an active-low reset
is input to the micro controller rather than an active-high reset.

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