我无法从x86汇编中的CD-ROM读取扇区 [英] I can't read a sector from a CD-ROM in x86 assembly

问题描述

我正在尝试从我的OS32操作系统的ISOSYS引导加载程序中加载第二阶段的引导加载程序，但是我似乎做不到.CF似乎总是被设置.这将从CD/DVD引导，我正在使用Debian GNU/Linux 7.6(Wheezy)

I'm trying to load the second stage bootloader from my ISOSYS bootloader for my OS32 operating system, but I can't seem to do it. CF seems to always be set. This is to be booted from a CD/DVD and I'm using Debian GNU/Linux 7.6 (Wheezy)

这是isosys.asm:

Here's isosys.asm:

;===============================================================
;
;               ISOSYS v1.0 in isosys.asm
;
; Program for booting OS32 and OS32-based systems from CD/DVDs
; using the ElTorito boot standard.
;
; By Omar, 2014
;
; ===============================================================

; set up bios...
bits 16
org 0x7c00

; beginning of sector 1, byte 0:

; jump over functions block
jmp Start

;
; Print()
;      prints a string to standard output
; AL = 0 : terminate
;

Print:
   lodsb            ; grab a byte from SI and place it in AL
   cmp al, 0         ; is AL = 0?
   jc PrintDone         ; yep, we're done so return
   mov ah, 0eh
   int 0x10         ; nope, print the character
   jmp Print         ; loop until no more characters to be printed
PrintDone:
   ret            ; return to wherever we were!

;
; Start()
;      bootloader entry point
;

Start:
   mov si, creditmsg
   call Print         ; print the first message
   mov dh, 1         ; read 1 sector

.ReadSector:
   cli            ; disable interrupts
   push dx            ; push this stack value
   sti            ; re-enable interrupts

   mov ax, 0x1000         ; read sector into address 0x1000
   xor bx, bx         ; read sector into offset 0x0
   mov ah, 0x02         ; read sector function
   mov al, dh         ; read 1 sector
   mov ch, 0x0         ; must be 0x0 to read sector
   mov dh, 0         ; reading head 0
   mov cl, 0x02         ; start reading from sector 2 to load the second stage

   mov [CDDriveNumber], dl      ; store our drive number is variable CDDriveNumber
   cli            ; disable interrupts
   pusha            ; save the stack
   sti            ; enable interrupts

   int 0x13         ; INT 13h
   jc Error         ; handle error because Carry Flag is set

   pop dx            ; Restore DX

.ExecuteSector:
   ; now that our sector was loaded into address 0x1000:0x0, let's execute it!
   jmp 0x1000:0x0         ; jump to execute the sector
;
; Error()
;      handles errors when carry is set
;

Error:
   mov si, errmsg
   call Print         ; print an error message

   cli            ; disable interrupts to prevent problems
   hlt            ; halt the system... :(

; data section

creditmsg db "ISOSYS v1.0 by Omar Mohammad, Init...", 0
errmsg db "Cannot load the second stage. (File not found maybe?)", 0
CDDriveNumber db 0

times 2048 - ($-$$) db 0      ; fill in 2048 bytes with zeroes

; end of sector 1! :D Any more code here would take bytes 2048 and 2049 but we don't    need that...

这是boot.asm，第二阶段:

This is boot.asm, the second stage:

bits 16
org 0x1000

Main:
   xor bx, bx
   mov ah, 0eh
   mov al, 'A'
   int 0x10

   cli
   hlt

times 2048 - ($-$$) db 0

我使用 nasm -f bin isosys.asm -o ISOSYS 和 nasm -f bin boot.asm -o BOOT 进行了编译然后我做了 mkdir iso ，然后是 cp ISOSYS iso/，然后是 cp BOOT iso/，然后生成了ISO: genisoimage -no-emul-boot -boot-load-size 4 -eltorito-boot ISOSYS -o boot.iso iso/

I compiled with nasm -f bin isosys.asm -o ISOSYS and nasm -f bin boot.asm -o BOOT Then I did mkdir iso and then cp ISOSYS iso/ then cp BOOT iso/ and then generated ISO: genisoimage -no-emul-boot -boot-load-size 4 -eltorito-boot ISOSYS -o boot.iso iso/

我在VirtualBox中运行它，但它只说是无法加载第二阶段(也许找不到文件?)，这意味着已设置了进位标志，但我似乎找不到错误.

I run it in VirtualBox but all it says is Cannot load the second stage (file not found maybe?) which means that Carry Flag is set but I can't seem to find the error.

有帮助吗?帮助将不胜感激！:*

Any help anyone? Help would be greatly appreciated! :*

推荐答案

您的堆栈处理已完全关闭.您将 dx (为什么?)推入 cli ‥ sti 块(WHY?)内，然后将其推入 pusha ，但不要't popa.

Your stack handling is totally off. You push dx (why?) inside a cli‥sti block (WHY?), then pusha but don't popa.

在 mov ax，0x1000 之后，您实际上还缺少一个 mov es，ax ，它实际上将段寄存器设置为要在其上加载代码的位置.

You're also missing a mov es,ax after the mov ax, 0x1000 to actually set the segment register to where you want your code to be loaded at.

您无法使用CHS功能(AH = 02h)读取ISO，必须将LBA功能(AH = 42h)与参数块一起使用.有一个例子.

You cannot read ISOs with the CHS function (AH=02h), you must use the LBA function (AH=42h) with a parameter block. Have an example.

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