Assembly Language for a stack machine

Resources

1. Stack of size 1000 elements each 4 bytes.
2. A boolean bit.
3. A program counter.

Example hello world program

# Hello world program __CODE__ # start code segment
# PUSH the string Hello World! in reverse order.
# 0 indicates end of string.
PUSH &end # the return address.
PUSH 0 PUSH 10 # new line.
PUSH '!'
PUSH 'D'
PUSH 'L'
PUSH 'R'
PUSH 'O'
PUSH 'W'
PUSH 32 # space
PUSH 'O'
PUSH 'L'
PUSH 'L'
PUSH 'E'
PUSH 'H'
PUSH Ah # new line. (in hexadecimal).
PUSH &print # call print function.
GOTO
:end
END
########## PRINT SUBROUTINE ###########
:print PUSH 0
EQU
POP # pop the 0.
PUSH &ret # If top of stack is zero stop printing.
GOIF
WRTC # print the character.
PUSH &print # loop.
GOTO
:ret
POP # POP the 0.
GOTO # TOS is return address, so go to it.
:end

Instructions without args

REAH - read a hexadecimal byte from stdin to top of stack
READ - read a decimal to top of stack.
REAC - read a character to top of stack.
WRTH - write a hexadecimal byte to stdout.
WRTD - write a decimal to stdout.
WRTC - write a character to stdout.
ADD - add top 2 elems of stack and put sum
in stack
SUB - subtract topmost element by 2nd topmost element from stack and put
difference in top of stack
MUL - multiply top 2 elems os stack and put product in top of stack
DIV - Divide topmost elem by 2nd topmost
elem and put quotient in top of stack and remainder below top.
POP - pop topmost elem from stack
EQU - if top 2 elems are equal boolean flag set to 1 else 0
GRT - is top is greater than second top boolean flag set to 1 else 0
LST - is top is smaller than second top boolean flag set to 1 else 0
GOTO - go to instruction number given by top of stack.
GOIF - like GOTO but only if boolean flag set.
GOUN - like GOTO but only if boolean flag not set.
END - end execution.
DUP - Duplicate the topmost element on the stack.
FLIP - Flip the top two elements on the stack.
GET - Push the element at data segment address given by the top of stack.
PUT - Put the second in top of stack to the location given by top of stack.
NOP - No operation.

Instructions with arguments

PUSH - push a byte to top of stack. Byte may
be entered as a decimal or a hexadecimal with a suffix 'h'.

Bytecodes used for compiler hints

LAB - Defines a label at this point.
IND - Indirection, defines that next 4 bytes
is an address.

In a second pass of compilation LAB is replaced by NOP and IND is compiled to PUSH .

Labels

A label can be defined with the ':' character, and can be jumped to with GOTO, GOIF or GOUN instructions, for example-

....code....
:start \# Definition of a label.
....code....
GOTO &start \# Jump to label 

To push data from data segment to stack, first push the address to the stack then use GET, for example-

:my_data 123
__CODE__ PUSH &my_data
GET

Similarly, PUT can be used to write data from the stack to the data segment.

comments

Use the # character for comments.

How to run the example programs

First compile file to byte code.

./compiler hw.vm

Then run the compiled .vmc file in the vm.

./vm hw.vmc

To decompile the code to a .vm file use decompiler.

./decompiler hw.vmc