Assembly language OS boot loader "using of nonexisting segment register 7" error

Question

I'm currently working on a basic OS for a university assignment. I am working in x86 assembly language. I seem to have an issue with switching to the second stage of my boot-loader, resulting in the error above when I try to run the application in Bochs. From my testing, I think that the error is a result of the application not reading the program from memory properly, either due to a mistake in the first stage of my boot loader or something being wrong in the makefile. I will include both of these below:

First Stage of bootloader:

BITS 16

ORG 7C00h
jmp     Real_Mode_Start 

%include "functions_16.asm"

Read_Failed:
  mov       si, boot_error
  call Console_WriteLine_16
  ret

Real_Mode_Start:
  cli                                   
  xor   ax, ax                       
  mov   ss, ax
  mov   sp, 4000h

  mov   ds, ax                      

  mov   si, boot_message            
  call  Console_WriteLine_16

  mov   al, 5                       
  mov       bx, 9000h                   
  mov       ch, 0                       
  mov       dh, 0                       
  mov       dl, 0 
  mov       cl, 2                       
  int       13h
  cmp       al, 5                       
  jne       Read_Failed

  jmp   9000h

  hlt                                   


; Data
boot_message:   db  'MacOS Remastered' , 0
boot_error:     db  'Boot Failed' , 0

times 510 - ($ - $$) db 0

dw 0AA55h

Makefile:

.DEFAULT_GOAL:=all
Imgname=MacRemastered
.SUFFIXES: .iso .img .bin .asm

%.bin: %.asm
nasm -w+all -f bin -o $@ $<

boot.bin: boot.asm functions_16.asm 
boot2.bin: boot.asm functions_16.asm

$(Imgname).iso: boot.bin boot2.bin
cp floppy_image/$(Imgname).img $(Imgname).img
dd status=noxfer conv=notrunc if=boot.bin of=$(Imgname).img
dd status=noxfer conv=notrunc seek=1 if=boot2.bin of=$(Imgname).img
rm -rf cdiso
mkdir cdiso
cp $(Imgname).img cdiso/$(Imgname).img
mkisofs -o $(Imgname).iso -b $(Imgname).img cdiso/  

all: $(Imgname).iso

clean:
rm -f boot.bin
rm -f boot2.bin
rm -f $(Imgname).img
rm -f $(Imgname).iso
rm -rf cdiso

Any help would be appreciated.

Would you mind giving us a hint as to which line it is reporting the error on? The bootsector that you have posted looks fine. It could very well be in the subsequent code you're loading. — David Hoelzer
– David Hoelzer, Commented Oct 4, 2016 at 23:34
Awesome, setting ah to 2 fixed it. I haven't even touched the mov dl, 0 part yet but will have a look at that next. — FloppyCatfish
– FloppyCatfish, Commented Oct 5, 2016 at 11:16

Michael Petch · Accepted Answer · 2016-10-05 17:12:58Z

You should review the docs for Int 13h/ah=2h. Ralf Brown's Interrupt guide is the bible of DOS and BIOS interrupts. The guide says this:

DISK - READ SECTOR(S) INTO MEMORY

AH = 02h
AL = number of sectors to read (must be nonzero)
CH = low eight bits of cylinder number
CL = sector number 1-63 (bits 0-5)
high two bits of cylinder (bits 6-7, hard disk only)
DH = head number
DL = drive number (bit 7 set for hard disk)
ES:BX -> data buffer

Return:

CF set on error
if AH = 11h (corrected ECC error), AL = burst length
CF clear if successful
AH = status (see #00234)
AL = number of sectors transferred (only valid if CF set for some BIOSes)

Most issues are related to your disk read:

You should set ES to 0 as the buffer is an address specified by ES:BX.
Don't set DL to zero. The BIOS will set DL to the boot drive before transferring to your code.
You didn't set AH to 2 which tells Int 13h that you want to do a disk read.
Rather than cmp al, 5 jne Read_Failed to test for disk error you can check the Carry Flag(CF). Replace both those lines with a simple jc Read_Failed right after Int 13h

I also recommend using the BOCHS debugger for 16-bit real mode debugging. When the BOCHS debugger starts set a breakpoint at 0x7c00 with the command b 0x7c00 then use the command c to continue. That should start the BOCHS startup and should break at the beginning of the boot sector. Basic commands:

help for a list of commands
n (next)
s (step)
c will continue until next break point.
b address breaks on an address. ie: b 0x7c00

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