[SOLVED]How to use the CMP instruction

bashcommando · **Joined:** Mon Jan 06, 2014 4:23 pm **Posts:** 62

I have a problem with my code. I don't know how to debug assembly. I have a problem with the

Code:

cmp

command. Here is the code:

Code:

    BITS 16

start:
    in al, 0x92
    test al, 2
    jnz skip
    or al, 2
    and al, 0xFE
    out 0x92, al
    jmp skip

skip:
    mov ax, 07C0h
    add ax, 288
    mov ss, ax
    mov sp, 4096
    mov ax, 07C0h
    mov ds, ax
    mov si, welcome
    call print_si
    jmp term
    welcome db 'Welcome to starfruit', 0

print_si:
    mov ah, 0Eh

.repeat:
    lodsb
    cmp al, 0
    je .done
    int 10h
    jmp .repeat

.done:
    ret
    times 510-($-$$) db 0
    dw 0xAA55

term:
    int 80h
    [color=#FF0000]cmp 80h, 0x65[/color]
    je .done
    [color=#FF0000]cmp 80h, 0x76[/color]
    je vdisplay
    [color=#FF0000]cmp 80h, 0x62[/color]
    je boothdd
    jmp error

error:
    mov si, emsga
    call print_si
    mov si, emsgb
    jmp term
    emsga db 'Commands are 1 byte long', 0
    emsgb db 'Try v, b, or e', 0

vdisplay:
    mov si, version
    call print_si
    jmp term
    version db 'Version 1.00', 0

boothdd:
    mov si, haltmsg
    call print_si
    hlt
    haltmsg db 'System Halt', 0

I highlighted the lines NASM told me was the problem. But I don't see a problem(I'm new to assembly).
This is how I build it:

Code:

nasm -f bin -o boot.bin boot.asm
dd status=noxfer conv=notrunc if=boot.bin of=floppy.flp
mkisofs -o disc.iso -b floppy.flp

Help?

AbstractYouShudNow · **Joined:** Tue Aug 14, 2012 8:51 am **Posts:** 92

You added code after the end. Everything after 0xAA55 is not included in the bootsector, so you jump to a location that contains random values, which causes an exception, which ultimately results in the pc rebooting

Bender · **Posted:** Tue May 20, 2014 7:56 am

Code:

int 80h

?

Quote:

cmp 80h, 0x65

So you're trying to check if 0x80 = 0x65? Do they?

The CMP instruction does exactly what it says it compares two operands and sets the (E/R)FLAGS accordingly. Like,

Quote:

cmp ax, 7 -- Is AX = 7?
If AX is equal to 7 IIRC the ZF (Zero Flag) will be set, and you can use JZ or JE.
JE/JZ AddressX -- Jump to AddressX if ZF set

Have a look at the x86 jump instructions here: http://www.unixwiz.net/techtips/x86-jumps.html
Similarly, you can use:

Quote:

cmp ax, [7] -- Is AX = value at address 7?

bashcommando · **Joined:** Mon Jan 06, 2014 4:23 pm **Posts:** 62

AbstractYouShudNow wrote:

You added code after the end. Everything after 0xAA55 is not included in the bootsector, so you jump to a location that contains random values, which causes an exception, which ultimately results in the pc rebooting

I at least know how to reboot it now. :lol:

eryjus · **Posted:** Tue Jan 20, 2015 5:40 pm

I recommend you take a look at this reference. It has everything you ever wanted to know about programming for an Intel processor, including the instruction set reference for CMP.

bashcommando · **Joined:** Mon Jan 06, 2014 4:23 pm **Posts:** 62

eryjus wrote:

I recommend you take a look at this reference. It has everything you ever wanted to know about programming for an Intel processor, including the instruction set reference for CMP.

Thanks but I already figured it out, you can't use cmp with 2 hexadecimal values without storing it in a variable.

Combuster · **Posted:** Wed Jan 21, 2015 10:08 am

Why would you even want to compare two constants in the first place?

bashcommando · **Joined:** Mon Jan 06, 2014 4:23 pm **Posts:** 62

Combuster wrote:

Why would you even want to compare two constants in the first place?

You see, a year ago I thought 0x80 was a memory location not a constant. I needed to use a interrupt, then again I wasn't using linux so int 80h would not have worked anyways.

OSDev.org

[SOLVED]How to use the CMP instruction

Who is online