Bios Extension boot and bootsector programs.

Last Updated or created 2023-07-12

Followup on

Today two boot projects.
One using a bios extension, so it chip based.
Second is a floppy disk boot program. (Creating a test situation to get our old Boot floppy demo working. ( That one without using an operatingsystem like ms-dos.

Creating a Secondary Bios ROM

NAME mycode
.model small
ORG 0h

	dw 0AA55h ;  Magic header your bios is looking for
	db 16     ; lenght of this rom in 512 bytes == 8k
	jmp short clear ; jmp to program

ORG 20h                 ; start of program
clear:  mov cx,10       ; clear, set keyboard led and print 10 # chars
	mov ah,0ah
	mov al,31h
	int 10h
	mov bh,0
	mov cx,1
start:	mov al, 11000000b
        out 80h, al
print:  mov cx,10
	mov ah,0ah
	mov al,"#"
	int 10h
loop1:  nop           ; loop until doomsday
	jmp loop1
	db -68        ; This makes the checksum 0
                      ; steps to take: edit source, make this byte entry 0
                      ; compile using make.bat in dosbox
                      ; check checksum using my python script
                      ; output was 68 hex 0x44
                      ; edit asm file place -68 to make the checksum 0x00 again
                      ; compile and burn to ROM
ORG 2000h             ; create end of rom 0000h-1fffh = 8K

make.bat in dosbox


Python script here:

Write EEprom

minipro -w 1.COM -p AT28C64

Part 2 – Bootsector program !

Allmost the same as above, but booting from a Floppy disk.

Video mode info :

Assembly code

use16              ; 16 bits 
org 0x7c00         ; start address ( change? )

mov ah,0x0         ; ah 0h - video mode 
mov al,0x0         ; al 0h - mode 0 - 40x25 chars
int 10h            ; scree routines 
mov cx,11h         ; 11 chars
mov ah,0ah         ; ah 0ah - print char mode
mov al,'#'         ; choose char as #
int 10h            ; execute 

times 510 - ($-$$) db 0 ; fill rest 512 bytes sector

dw 0xaa55 ; magic bytes 

Leave a Reply

Your email address will not be published. Required fields are marked *