simplecpu, a simple CPU architecture, emulator and assembler in C99, ~1000 LOC.

C99, no dependencies other than libc and POSIX. Doesn't use lexx/flex or yacc/bison for lexing / parsing.

Directories

• scripts contains example .ASM files to run.

Building

• POSIX (Linux [x64 and Raspberry Pi]), OSX, Cygwin)

• ./make.sh

• Windows (native, Visual Studio 2015 Community).

• VS\simplecpu.sln

Running

• simplecpu asmfilename [Y]

Assembles and runs specified file. Currently no support for pre-assembled memory images. Y being optional debug output from assembler.

Assembly language syntax

; Comment

:LABEL Label

12345 Literal number

'A' Literal ASCII character (eg 65)

:BOTTLES "bottles of beer" Labelled string

#include Include file, eg: #include "../scripts/incl.asm"

Target operand is first, eg: ADD REG LIT will add literal LIT to register REG.

Instructions:

• HLT HALT (stop CPU)
• NOP No operation
• INC REG Increment register, eg INC R10
• DEC REG Decrement register, eg DEC R10
• ADD REG (LIT|REG) Add literal or register value to register
• SUB REG (LIT|REG) Subtract literal or register value from register
• MUL REG (LIT|REG) Multiply register value by literal or register value
• JMP :LABEL Unconditional jump to :LABEL
• JEQ :LABEL Jump to :LABEL if equal
• JNQ :LABEL Jump to :LABEL if not equal
• JGT :LABEL Jump to :LABEL if greater than
• JLT :LABEL Jump to :LABEL if less than
• MOV REG (LIT|REG|PORT) Copy value from register, literal or port to register, eg: MOV R9 $INPORTC
• MOV PORT REG Copy value from register to port, eg: MOV $OUTPORTC R10 or MOV $OUTPORTI R10
• CMP REG (LIT|REG) Compare and set flags
• PUSH (LIT|REG) Push value or register value to stack
• POP REG Pop value to specific register
• CALL Call :LABELNAME (Subroutine call)
• RET Return from subroutine
• PUSHU Push all user registers to stack, (not SP or PC).
• POPU Pop all user registers from stack, (not SP or PC).
• IRQ NUM Raise IRQ
• MOD REG LIT Modulo division, leave remainder in REG.
• MOD REG REG Modulo division, leave remainder in REG.

Additional Assembler instructions:

Not CPU instructions, but commands for assembler.

• REBASE Choose different memory location to continue assembling from.

CPU Flags:

• EQ Equality flag
• C Carry flag

Supported PORTS:

• $INPORTC Input character

• $INPORTI Input integer

• $OUTPORTC Print output character

• $OUTPORTI Print output integer

• $I00 - $I15 Interrupt vectors

VARIABLES

$XYZABC Assign label "XYZABC" to a word of storage.

Interrupts

$I00 to $I15 are the interrupt vectors.

IRQ0 is KEYBOARD interrupt, eg:

; Setup interrupt handler (IRQ0)
MOV $I00 500			; ie jump to location 500 to service IRQ0 (keyboard)

REBASE 500				; Move assembly point to location 500
MOV R9 $INPORTC			; Get character
CMP R9 'X'				; See if it's eXit
JEQ :INPEND				; Jump to "end of input"
MOV $OUTPORTC R9		; Print value
:INPEND					; "End of input" label
RET						; Return

Register use

16 registers.

R0 = PC

R1 = SP

R2 = Flags (Not yet supported)

R3 to R9 = GP registers.

R10 = First CALL register

R11 = Second CALL register

R12 = Third CALL register

R16 = RETURN value register

Stack

Stack starts at nearly top of memory (STACKTOP) and grows downwards. - No stack overflow checks.

Variables

Declare word variables as: $NUMBOTTLES

Declare static string variables as (for instance):

:TAKEONE " take one down, pass it around, now there's "

TODO

• Terminal control is incomplete for all platforms, so monitor.asm will not work (and is incomplete itself)
• Interrupt driven I/O needs work
• Would like to simplify instruction set further, possibly make this more 6502-like.
• Not tested on Linux / MacOS / BSD