let me preface this by explaining the reason i'm writing this code.. i've been writing an x86 PC emulator in FreeBASIC, and this program is designed to test it's handling of video mode 13h, including palette manipulation. i thought in addition to being fun (at least for a nerd like me), writing an 8086 CPU + PC architecture emulator from the ground up would be the ultimate crash course in teaching myself assembly language and understanding the PC's internals better. i've been on this project for about 3 months now.
anyway... i've written this code that has the data of a 320x200 8-bit photo of a stupid looking goat embedded in it, as well as the palette data. it compiles to a .COM file using MASM 6.11.. however, the output it draws to the screen looks all messed up. this is it running in the emulator i'm coding. (it's not a bug in my emu, it looks exactly the same if i use DOSBox)
http://rubbermallet.org/goat1.png
i am not going to embed the whole source file in this post, because it's over 300 KB. (99.9% of that is just the raw image data after the code section) but here is the actual code lines:
.8086
.MODEL tiny
.CODE
start:
org 100h ;we want to make a .com so align it after the DOS PSP
mov ax, 13h ;function 0, AL = 13h
int 10h ;on int 10h to set 320x200 256-color mode
push cs ;make sure our DS is the same
pop ds ;as our CS
mov ax, word ptr [pal] ;place the pointer to the palette
mov si, ax ;data start point into SI
mov dx, 3C6h ;VGA palette mask register
mov al, 0FFh ;mask all
out dx, al ;send that byte!
mov dx, 3C8h ;VGA palette index register
mov al, 0h ;we want to start at entry 0
out dx, al ;send that byte!
mov dx, 3C9h ;the palette data bytes will all go to this port
mov cx, 768 ;256 colors to set, 3 bytes of data each
nextpal:
lodsb ;load current byte pointed to by SI into AL
out dx, al ;and send it to the VGA palette data port
inc si
loop nextpal
mov ax, word ptr [image] ;place the pointer to the image
mov si, ax ;data start point into SI
mov ax, 0A000h ;set the VGA base segment
mov es, ax ;and put it into ES for our movsw cycles
mov di, 0h ;and make sure we start at offset 0
mov cx, 32000 ;64,000 bytes, but moving 2 at a time
rep movsw ;DRAW THAT GOAT!!
mov ax, 0h ;finished, wait for the user
int 16h ;to press a key
mov ax, 3h ;before we go back into 80x25 16-color text mode
int 10h
ret
.DATA
image db ;SNIP! the next few thousand lines in the code is just the raw image data db'd
.DATA
pal db ;and the next 256 lines from here are RGB triplets db'd
end start
ENDif you want the entire file, it's here:
http://rubbermallet.org/goat.asm
any idea why it looks the way it does? i'm sure the answer is obvious to somebody more experienced.
thanks!
-mike
