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Guide for Python-Assembler

Register

In this programming language you can use four registers.

  • eax
  • ebx
  • ecx
  • edx

The register eax will be standard use for multiplication and division. Commands for arithmetic are:

  • add p0, p1
  • sub p0, p1
  • mul [register]
  • div [register]

p0 and p1 stands for parameter. p0 must be a register, p1 can be a register or constant. The commands mul and div standard use eax. For instance:

mov ecx, 56
sub ecx, 10
mov eax, 4
int 0x80
  • The first line move the number 56 into register ecx.
  • The second line subtract 10 from the ecx register.
  • The third line move the number 4 into the eax register. This is for the print-function.
  • The fourt line call interrupt 0x80, thus the result will print onto console.
  • The fifth line is a new line. This is important.

Important: close each line with a newline!

System-Functions

With the interrupt 0x80 you can use some functionality in your program.

EAX Function
1 exit program. error code in ebx
3 read input. onto ecx (only float)
4 output onto console. print content in ecx

EAX stands for the register eax

Variables

Variables begin with a $ or written in uppercase.

For instance:


; variables
VAR1 db 56
$var1 db 10

mov ecx, VAR1
mov ebx, $var1
sub ecx, ebx
mov eax, 4
int 0x80

Important: The arithmetic commands (add, sub) works only with registers or constans. Therefore we must use the register ebx as a placeholder, above.

Result of code, above.

46

Comments

Comments begin with ; and ends with a newline. We noticed a comment, above.

Push and Pop

Sometimes we must save the content of a register, against losing of data. Therefor we use the push and pop command.

push eax

This line will push the content of register eax onto the stack.

pop ecx

This line will pop the content of the top of the stack onto the ecx register.

push [register]
pop [register]

Jumps

With the command cmp we can compare two register.

cmp r0, r1
je l1
jmp l2

Are the two register equal? The the command je is actively and jumps to label l1 Otherwise the command jmp is actively and jumps to label l2

Labels

For instance

l1:

is a label. Labels begin with a l and contains numbers. For instance l1, l2 etc ...

To set a label you must end with a colon. If you use a label in the jump commands, then avoid the colon at the end.

Subprograms

mov ecx, 5

call _double
call _cube
call _inc

mov eax, 4
int 0x80
mov eax, 1
mov ebx, 0
int 0x80



_double:
add ecx, ecx
ret 

_cube:
push eax
mov eax, ecx
add ecx, eax
add ecx, eax
pop eax
ret

_inc:
add ecx, 1
ret

A subprogram label begins with a _ and ends with a colon. See above.

If you call the subprogram you must avoid the colon.

Important: Each subprogram must end with the ret command.