CODEWITHSUNDEEP
assemblymemorynasmdosx86-16
pandoragami
pandoragami

Reputation: 5585

16-bit x86 assembler, trouble drawing blue to screen using 8 -bit memory stored variable for color?

The following code compiles and runs fine on Xubuntu 16.04 with these commands in bash shell

nasm blue.asm -fbin -oblue.com

dosbox ./blue.com -exit

The problem I'm having is on line 20 mov al, 1;byte [blue]

where if I use this instead mov al, byte [blue]

the program draws a sort of burgundy to the screen instead of blue. It works normally for using 1 which is the color code in the 8-bit palette here https://en.wikipedia.org/wiki/BIOS_color_attributes

Here's the full code, feel free to let me know if anything else is wrong with it though.

org 00h
bits 16       

section .data             

    blue:    db       1  

section .text
MAIN:

AsyncKeyInput:      

    mov     al, 13h
    int     10h
    ; Segment a000h
    mov     ax, 0a000h
    mov     es, ax
    ; Offset 0
    xor     di, di
    mov     al, 1;byte [blue]
    ; Looplength (320*200)/2 = 7d00
    mov     cx, 7d00h

hplot:
    mov [ es: di], al       ;set pixel to colour
    inc di          ;move to next pixel
    loop hplot      

    mov     ah, 1          ;Get the State of the keyboard buffer
    int     16h            ;key press
    jz      AsyncKeyInput ;if not zero then exit the program 

    ;exit program
    mov eax, 1
    mov ebx, 0
    int 0x80
ret

Upvotes: 2

Views: 156

Answers (1)

pandoragami
pandoragami

Reputation: 5585

The solution to the problem is to properly set the program segment prefix to org 100h for com programs. Here's the corrected code below.

org 100h
bits 16       

section .data             

    blue:    db       1h     

section .text
MAIN:

AsyncKeyInput:      

    mov     al, 13h
    int     10h
    ; Segment a000h
    mov     ax, 0a000h
    mov     es, ax
    ; Offset 0
    xor     di, di
    xor     eax, eax
    mov     al, byte [blue]
    ; Looplength (320*200)/2 = 7d00
    mov     cx, 7d00h

hplot:
    mov [ es: di], al       ;set pixel to colour
    inc di          ;move to next pixel
    loop hplot      

    mov     ah, 1          ;Get the State of the keyboard buffer
    int     16h            ;key press
    jz      AsyncKeyInput ;if not zero then exit the program 

    ;text mode
    mov     ax, 0003h
    int     10h         
ret

Upvotes: 2

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