assembly - NASM assembled bootloader memory issue -
i writing bootloader nasm. @ moment designed output welcome string, record keystrokes while displaying them, printing stored keystrokes upon finding enter key, , halting.
bits 16 org 0x7c00 start: jmp main bgetkey: pusha mov ax, 0 mov ah, 10h int 16h mov [.buf], ax popa mov ax, [.buf] ret .buf dw 0 prints: mov ah, 0x0e mov al, [si] cmp al, 0 jz print_end mov bh, 0x00 mov bl, 0x07 int 0x10 inc si jmp prints print_end: ret main: mov ax, 0x0000 ; set register mov ds, ax ; mov bx, mem add bx, word 1 mov word [mem], bx mov si, welcome ; set , prints call prints ; type: mov si, qbuf ; set prints ptr call bgetkey ; capture input mov [qbuf], al ; set char sz call prints ; call print str mov bx, [mem] ; put chr in mem cmp bx, stop ; compare loader je oom ; end , memory mov byte [bx], al add bx, byte 1 mov [mem], bx ; cmp byte [qbuf], 0x0d ; cmpr enter key jne type ; continue next mov si, newline ; print newline call prints ; mov bx, mem printmem: cmp byte [bx], 0x00 ; check 0 je halt ; halt cpu mov cl, [bx] mov byte [qbuf], cl ; buffer , cpy mov si, qbuf ; pointer si call prints ; print char inc bx jmp printmem ; jump beginning oom: mov si, outomem ; no more memory call prints ; print message halt: mov si, halting ; cpu halting call prints ; print last msg hlt ; halt cpu welcome db "bootloader", 0x0a, 0x0d, 0x00 newline db 0x0a, 0x00 outomem db "out of memory", 0x0a, 0x0d, 0x00 halting db "halting", 0x00 qbuf dw 0, 0 mem db 0 times 0200h - 2 - ($ - $$)db 0 stop dw 0xaa55 the program not functioning desired. ceaselessly prints same character after enter pressed. how error corrected?
the immediate problem prints destroys bx (because sets bl , bh) printmem loop requires bx preserved blows up.
however, destroys al input loop won't storing correct value in memory start with, either.
furthermore, while want mem pointer mem + 2, pointer mem + 1 overwrite pointer input. also, start printing mem , not mem + 2.
finally, input not terminated 0 checking for, it's terminated 0x0d (the enter).
a working version be:
bits 16 org 0x7c00 start: jmp main bgetkey: pusha mov ax, 0 mov ah, 10h int 16h mov [.buf], ax popa mov ax, [.buf] ret .buf dw 0 prints: pusha .loop: mov ah, 0x0e mov al, [si] cmp al, 0 jz print_end mov bh, 0x00 mov bl, 0x07 int 0x10 inc si jmp .loop print_end: popa ret main: mov ax, 0x0000 ; set register mov ds, ax ; mov bx, mem add bx, word 2 ; point after pointer :) mov word [mem], bx mov si, welcome ; set , prints call prints ; type: mov si, qbuf ; set prints ptr call bgetkey ; capture input mov [qbuf], al ; set char sz call prints ; call print str mov bx, [mem] ; put chr in mem cmp bx, stop ; compare loader je oom ; end , memory mov byte [bx], al add bx, byte 1 mov [mem], bx ; cmp byte [qbuf], 0x0d ; cmpr enter key jne type ; continue next mov si, newline ; print newline call prints ; mov bx, mem+2 ; start after pointer printmem: cmp byte [bx], 0x0d ; check end je halt ; halt cpu mov cl, [bx] mov byte [qbuf], cl ; buffer , cpy mov si, qbuf ; pointer si call prints ; print char inc bx jmp printmem ; jump beginning oom: mov si, outomem ; no more memory call prints ; print message halt: mov si, halting ; cpu halting call prints ; print last msg hlt ; halt cpu welcome db "bootloader", 0x0a, 0x0d, 0x00 newline db 0x0a, 0x00 outomem db "out of memory", 0x0a, 0x0d, 0x00 halting db "halting", 0x00 qbuf dw 0, 0 mem db 0 times 0200h - 2 - ($ - $$)db 0 stop dw 0xaa55 ps: learn use debugger.
Comments
Post a Comment