Last Updated or created 2023-09-12
This is a work in progress, below are my Lab notes.
I want to rewrite pieces we made for a demo, loading images and effects from a floppydisk bootloader.
Without looking at old code (which was written using Masm), I wanted to learn the steps using Fasm.
I started with a boot sector program, It should do the following.
- Set graphic mode, and start a trackloader
- Load sector 2 and 3 which contains the color palette for the image.
- Next sectors, cylinders and heads contain the raw image
I got it working, half that is.
In the past I used real disks, and now a virtual disk, maybe thats the difference?
use16 org 0x7c00 mov ah,0x0 mov al,0x13 int 10h mov ax, 0a000h mov es, ax xor di, di mov ax, 50 mov cx, 64000 rep stosb loophere: jmp loophere times 510 - ($-$$) db 0 dw 0xaa55
Compiling and starting:
fasm mybootblock.asm qemu-system-x86_64 --drive format=raw,file=mybootblock.bin
This works, it sets the graphical mode and clears the screen.
Skipping the int 25h version
## Track read part xor ax, ax ; DS = 0 mov ds, ax cld mov ah, 2h ; int13h function 2 track read mov al, 2 ; number of tracks ( should be 2 for reading only palette) mov ch, 0 ; from cylinder number 0 mov cl, 2 ; the sector number 2 - second sector (starts from 1, not 0) mov dh, 0 ; head number 0 xor bx, bx ; BX = 0 mov es, bx ; ES = 0 mov bx, 7e00h ; Offset from above int 13h call setpal ## End part with setpalette routine ## appending palette.colors ## and a raw image setpal: mov dx,3c8h xor al,al mov di, ax out dx,al inc dx mov cx,256*3 mov si,07e00h rep outsb ret times 510 - ($-$$) db 0 dw 0xaa55 include 'palette.colors' times 2048 - ($-$$) db 0 file 'image.raw'
Seems there is still a header on the RAW file, lets look at how I made this.
NOTE! .. Below converts an image with a STANDARD VGA palette, not a custom one as used above
- Load a image in Gimp
- Resize to 320×200
- Import the standard VGA palette from:
- Gimp menu Image > Mode > Indexed and select your just imported palette. (Do not remove unused colors)
- Flip vertical (bmp writes image upside/down)
- Export as BMP Compatibility Options > Do not write color space information
Looking with ghex at the file I saw that there was a header 0x415 bytes large.
(Probably still palette colors in there)
dd if=shoes.bmp of=cutshoe.bmp bs=1 skip=1078 (0x415h + 3?)
worked for me
Loading the extra tracks didn’t work for me?!?!
But how could I define tracks/sectors and heads on a virtual floppy?
I tried to write sectors using debug.com
start dosbox imgmount a: /tmp/floppy.img -t floppy debug.com bootsector.bin -r bx 01 -r cx 512 ; set bx:cx for size -w 100 0 0 1 ; write from address 100, drive 0 (a), sector 0, number of sectors ; testing -l 100 0 0 1 ;load sector to addr 100 drive 0 sector 0 number of sectors This used to work with real disks on a real machine, not in dosbox ?!?! my way to create a disk in linux dd if=bootblock.bin of=disk1.img bs=512 count=1 seek=0 dd if=palette.col of=disk1.img bs=512 count=1 seek=1 # or 2? dd if=shoes.raw of=disk1.img bs=512 count=10000 seek=17
It looks like I can’t read futher than 18 sectors on a virtual floppy.
What next? Head=1? Cylinder=1?
Below the info from a floppy image before altering.
DOS/MBR boot sector, code offset 0x3c+2, OEM-ID “MSDOS5.0”, root entries 224, sectors 2880 (volumes <=32 MB), sectors/FAT 9, sectors/track 18, serial number 0x1c2a0d0f, unlabeled, FAT (12 bit), followed by FAT
Appending the RAW to a executable gave me problems to.
(Without making a bootdisk)
Above and below weird data. Appending the data to the executable needs some work also.
At least for today let me display this image correctly 🙂
use16 org 0x100 ; set mode 320x200 256 colors palette mov ah,0x0 mov al,13h int 10h ; clear screen routine, not really needed clearscreen: push ax mov ax, 0a000h mov es, ax pop ax xor di, di inc ax mov cx, 64000 ; 320x200 rep stosb ; call file loader call Loadfile ; after displaying the image or displaying an error, wait for keypress to exit waitforkeyloop: MOV AH,1 INT 16h JZ waitforkeyloop XOR AH,AH INT 16h Exit: MOV AX,3 ; default text mode 3 INT 10h MOV AX,4C00h ; exit to dos (terminate process) INT 21h Loadfile: MOV DX,filename MOV AX,3D00h ; open filehandle INT 21h JC Err1 MOV BX,AX ; filehandle MOV CX,0FFFFh ; size mov dx,0a000h ; destination 0000:a000h - Screen memory mov ds,dx MOV DX,0 MOV AH,3Fh ; read from file INT 21h JC Err1 MOV AH,3Eh ; close filehandle INT 21h RET ; print error Err1: push cs ; make ds same as cs pop ds MOV DX,TxtErr1 ; error MOV AH,09h INT 21h RET filename DB "shoes.bmp",0 TxtErr1 DB "shoes.bmp not found!",7,10,13,"$"
Tomorrow .. back to the track loader