I am writing a bootloader in x86-16 assembly on a floppy disk, meaning that I will have to read from the disk in order to load more of my code, however, every time I attempt to read the disk and check the disk status code, I always get 0x04 - sector not found. I have attempted to read with CX set to 0x0000, 0x0001, and 0x0101, but I don't really know what to do, or what I am doing wrong.
INT 13,1 - Disk Status Codes
INT 13,2 - Read Disk Sectors
BITS 16
%include "C:\x86asm\nasm\nasm.asm"
;%macro pad 1-2 0
; times %1 - ($ - $$) db %2
;%endmacro
;%idefine PUSHW PUSH STRICT WORD
;%idefine PUSHD PUSH STRICT DWORD
[org 0x7C00]
; https://stanislavs.org/helppc/ ;; BIOS
; https://c9x.me/x86/ ;; Instruction Set
; https://wiki.osdev.org/ ;; OS Development
; absolute address = (segment << 4) + address
; to simulate NES/SNES style memory mapping (banks), only use bits[9..15] ($x000)
JMP 0x0000:_start ; ensure CS = $0000
_start:
CLI ; clear interrupt flag (disable
; interrupts, opposite of 6502)
CLD ; clear direction flag
PUSH CS
POP DS ; prepare for puts / puthexbyte
PUSH CS
POP SS ; set up stack segment
MOV SP, 0x7C00
;; set graphics ;;
MOV AX, 0x0012 ; set screen mode
INT 0x10
MOV AH, 0x0B
MOV BX, 0x0201
INT 0x10
MOV DX, 0x0D22 ; display "LOADING..."
MOV BX, 0x0007
MOV SI, loadstr
CALL putsl
MOV AH, 0x86 ; wait a moment...
MOV CX, 0x000F
INT 0x15
;; load floppy disk ;;
MOV BP, 0x0800 ; if fail, read x times
.loadfailure:
SUB BP, 0x0100 ; decrement # of tries left
MOV AH, 0x02 ; print # of tries left
XOR DX, DX
INT 0x10
MOV DX, BP
CALL puthexbyte
MOV AH, 0x00 ; reset disk system
MOV DL, 0x00
INT 0x13
MOV AX, 0x0201 ; read
MOV CX, 0x0101
XOR DX, DX
PUSHW 0x1800 ; write to $18000 ~ $1FFFF
POP ES
XOR BX, BX
INT 0x13
PUSHF
MOV DH, AH ; show error code
CALL puthexbyte
POPF
JNC .loadsuccess
TEST BP, BP
JNE .loadfailure
MOV DX, 0x0D0F ; read fail;
MOV SI, badload ; print msg
CALL putsl
MOV AH, 0x86 ; wait
MOV CX, 0x001E
INT 0x15
INT 0x18 ; boot windows
.loadsuccess:
MOV DX, 0x0D0F ; read success;
MOV SI, nosystem ; DOS not finished,
CALL putsl ; tell user
MOV AH, 0x86 ; wait
MOV CX, 0x001E
INT 0x15
JMP 0x1000:0x8000 ; boot test
putsl:
; [DX] : (Y,X)
; (AH)
MOV AH, 0x02
INT 0x10 ; set cursor position
puts:
; [DS:SI] : String
; (AX, BX, SI)
MOV AH, 0x0E ; teletype mode
MOV BX, 0x000F ; set white text attribute
.putsloop:
LODSB ; load character from [DS:SI++]
TEST AL, AL ; check if NULL (x86 MOV does not
JE .endputs ; change zero-flag unlike 6502 LDA/LDX/LDY)
INT 0x10 ; print character
JMP .putsloop ; loop until NULL
.endputs:
RET
puthexbyte:
; [DH] : Value
; (AX, DH, BX)
MOV AH, 0x0E
MOV BX, asciihex
MOV AL, DH
SHR AL, 4
XLAT
MOV BX, 0x000F
INT 0x10 ; print character
MOV BX, asciihex
MOV AL, DH
AND AL, 0x0F
XLAT
MOV BX, 0x000F
INT 0x10
RET
asciihex:
db "0123456789ABCDEF", 0
loadstr:
db "LOADING...", 0
badload:
db "Unable to load disk. Attempting to load Windows...", 0
nosystem:
db "Operating System is incomplete. Loading Windows...", 0
tststr:
db "Disk read success. INT 0x20 set success. Jump success.", 0
pad 0x01FE ; pad to end of boot sector
dw 0xAA55 ; floppy disk boot sector signature
; code
PUSHW 0x0000 ; test setting INT
POP DS ; (tested; works when in
MOV [4 * 0x20], DWORD putsl ; boot sector)
MOV DX, 0x0D0D ; test INT by
MOV SI, tststr ; printing
INT 0x20 ; string
MOV AH, 0x86 ; wait
MOV CX, 0x001E
INT 0x15
INT 0x18 ; boot windows
pad 0x0400 ; pad to end of sector 1
;incbin "os.bin"
pad 0x00167FFF ; pad to disk end
db 0x00
Edit:
An explanation on how to convert disk sectors and tracks (ie. CX in INT 13,2) into a "linear address" would be greatly appreciated, as the method I am using to get my code onto a floppy disk has me opening the program HxD and manually copying and pasting my binary onto the the disk.
Also, the disk I am using is 'unformatted' (as far as Windows is concerned).
Also, also, if this changes anything, my PC's BIOS is (msinfo32)"American Megatrends Inc. 5.35, 2008-12-16."
