Issues with general purpose registers

Question

Hey everyone! I've wrote a simple Assembly script to print a list of numbers from 0-6 in the console. But once I'm still learning to use the language, for a better practice, I focused on using nothing but registers to hold data, using the eax register as the list range, and the ebx register as an indexer. Such usage seemed to be ok to me.

But after assembling and linking the code, I tried to run the .exe file and got a totally different output.

Here's the source-code:

main PROC

LOCAL n:DWORD

mov eax, 6 ; Works if eax is replaced for the "n" variable
mov ebx, 0

COMPARE:
cmp ebx, eax ; Works if eax is replaced for the "n" variable
jl FNC
jg BREAK

FNC:
print str$(ebx)
print chr$(13, 10)
jmp INCECX

INCECX:
inc ebx
jmp COMPARE

BREAK: ret

main ENDP

The logically expected output was:

0
1
2
3
4
5
6

But instead the code is generating multiple different outputs:

- Using eax as indexer:
- Output:
0
1
2

- Using ecx as indexer
- Output:
0

- Using edx as indexer
- Output:
0
1
2
3
4
5
6
7
8
... and so on

As much as my knowledge allows me to assume, these registers are for general purpose, so I find a lot strage that these aren't holding values correctlly.

I'm using MASM v6.14.8444 and MASM Incremental Linker v5.12.8078

I'd be thankful if you could help me on this. Thanks.

Answer 1

print isn't a hardware instruction. It generates code using registers to make a system call clobbering your values. I've not played with masm so can't tell you what it uses and my attempt to search it came up empty.