I'm just approaching to machine-level x86 coding, so please excuse the triviality of my question. The following code is intended to be a simple bootloader. It dumps some sector of floppy disk into memory and then it jumps to loaded code. In the loaded code I was trying to read from a memory variable, without success, as described in comments.
[ORG 0]
jmp 07C0h:start ; Goto segment 07C0
start:
; Update the segment registers
mov ax, cs
mov ds, ax
mov es, ax
reset: ; Reset the floppy drive
mov ax, 0
mov dl, 0
int 13h
jc reset
read:
mov ax, 1000h ; ES:BX = 1000:0000
mov es, ax
mov bx, 0
mov ah, 2 ; Load disk data to ES:BX
mov al, 5 ; Load 5 sectors
mov ch, 0 ; Cylinder=0
mov cl, 2 ; Sector=2
mov dh, 0 ; Head=0
mov dl, 0 ; Drive=0
int 13h ; Read!
jc read ; on error
jmp 1000h:0000 ; Jump to the program
times 510-($-$$) db 0
dw 0AA55h
; == Loaded code from second floppy sector ==
prog:
mov ah, 0x0E ; Prints a char. This one works: the '-'
mov al, '-' ; is printed.
mov bx, 0
int 10h
mov bx, 0
a:
mov al, [L1+bx] ; Should read from L1 and print out chars.
inc bx ; But it prints only white spaces. Why?
int 10h
cmp bx, 10
jz h
jmp a
cli
hlt
L1 db "0123456789" ; my string
I can't understand why it doesn't work. I very appreciate any help.
The "Loaded from floppy" part was compiled for a different base offset than the one it was loaded into, you need to recalculate its address
You could also use ORG 7C00h and spare yourself the jump, the difference being your segments will be 0 instead of 07C0h
You can either calculate the new offset as [L1-prog] or you can reorder your code:
jmp 1000h:000Ah ; Jump to the program
....
L1 db "0123456789" ; my string
prog:
mov ah, 0x0E
...
mov al, cs:[bx]
I don't have an assembler to test but you get the idea - remove the need for a floating address, put it in the beginning
I roughly estimate your actual offset in segment 1000H to be around 32-ish, that's where your translated L1 will roughly be. Instead your L1 is calculated compile time to be about-ish 550, so you're actually trying to load something from the second sector you read. Do you have some blank characters or zeroes in the beginning of the second loaded sector?