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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