Set BPB data from the BIOS, properly support loading data from sectors >63, calculate CHS addressing properly. Calculate certain important values rather than harcoding them.

boot.asm

@@ -1,24 +1,28 @@
-[BITS 16]  ;tell the assembler that its a 16 bit code
+[BITS 16]
 
 jmp short bootcode
 nop
 
 bpOEMid db "XNOE    "
+
 bpBytesPerSector dw 512
 bpSectorsPerCluster db 1
 bpReservedSectors dw 1
 bpNoFATs db 2
 bpRootDirEntries dw 256
-bpLVSectors dw 8419
+bpLVSectors dw 8586
-bpMediumID db 0F0h
+bpMediumID db 0xF8
-bpSectorsPerFat dw 9
+bpSectorsPerFat dw 34
 bpSectorsPerTrack dw 18
-bpSides dw 2
+bpHeads dw 2
-bpHiddenSectors dd 0
+bpHiddenSectors dw 0
-bpLargeSectors dd 0
+
-bpDriveNo dw 0
+TIMES 36 - ($ - $$) db 0
-bpSignature db 41
+
-bpVolumeID dd 00000000h
+bpDriveNo db 0
+db 0
+bpSignature db 0x29
+bpVolumeID dd 0x0
 bpVolumeLabel db "XNOE OS    "
 bpFileSystem db "FAT16   "
 
@@ -26,7 +30,18 @@ bootcode:
   mov ax, 7c0h
   mov ds, ax
 
-  mov byte [drive], dl
+  mov byte [bpDriveNo], dl
+
+  ; Get the disk configuration from the BIOS
+  mov ah, 08h
+  int 13h
+
+  add dh, 1
+  movzx dx, dh
+  mov word [bpHeads], dx
+  and cl, 03fh
+  movzx cx, cl
+  mov word [bpSectorsPerTrack], cx
 
   mov si, boot_msg
   call _boot_print
@@ -35,12 +50,13 @@ bootcode:
   mov es, ax
   mov bx, buffer
 
-  mov ah, 2
+  ; Calculate position of rootdirentries
-  mov al, 16
+  mov ax, word [bpSectorsPerFat]
-  mov cl, 20
+  movzx cx, byte [bpNoFATs]
-  mov ch, 0
+  mul cx
-  mov dh, 0
+
-  mov dl, byte [drive]
+  add ax, 2
+  call prep_i13
 
   int 13h
 
@@ -57,11 +73,6 @@ kernel_finder:
   mov di, buffer
   add di, ax
 
-;  mov si, di
-;  call _boot_print
-;  mov si, new_line
-;  call _boot_print
-
   mov si, kernel_file
   mov cx, 11
   rep cmpsb
@@ -78,6 +89,7 @@ kernel_finder:
   jmp $
 
 kernel_found:
+;  jmp $
   mov ax, [es:di+0fh]
   mov word [cluster], ax
 
@@ -86,29 +98,41 @@ fat_loader:
   mov es, ax
   mov bx, buffer
 
+  mov ax, word [bpSectorsPerFat]
   mov ah, 2
-  mov al, 9
   mov cl, 2
   mov ch, 0
   mov dh, 0
-  mov dl, byte [drive]
+  mov dl, byte [bpDriveNo]
 
   int 13h
 
 kernel_loader:
+
+  ; Calculate file offset
+  mov ax, word [bpSectorsPerFat]
+  movzx cx, byte [bpNoFATs]
+  mul cx
+
+  push ax
+  mov ax, word [bpRootDirEntries]
+  mov cx, 16
+  div cx
+
+  mov bx, ax
+  pop ax
+  add ax, bx
+
+  mov word [fileoffset], ax
+
   mov ax, 2000h
   mov es, ax
   mov bx, word [pointer]
 
-  mov al, byte [cluster]
+  movzx ax, byte [cluster]
-  add al, 34
+  add ax, word [fileoffset]
 
-  mov cl, al
+  call prep_i13
-  mov al, 1
-  mov ah, 2
-  mov ch, 0
-  mov dh, 0
-  mov dl, byte [drive]
 
   int 13h
 
@@ -117,7 +141,6 @@ kernel_loader:
   mov si, word [cluster]
   shl si, 1
   add si, buffer
-;  add si, 1
   cmp word [si], 0ffffh
   je kernel_loaded
 
@@ -125,10 +148,6 @@ kernel_loader:
   jmp kernel_loader
 
 kernel_loaded:
-  ; Pass boot device to the kernel
-  xor ax, ax
-  mov al, byte [drive]
-  push ax
   jmp 2000h:0h
 
 _boot_print:
@@ -149,11 +168,34 @@ kernel_nf db "KERNEL.BIN Missing!", 13, 10, 0
 new_line db " ", 0
 kernel_file db "KERNEL  BIN"
 
+fileoffset dw 0
+
 cluster dw 0
 pointer dw 0
-drive db 0
 
-TIMES 510 - ($ - $$) db 0    ;fill the rest of sector with 0
+; AX set to the logical sector number.
-DW 0xAA55          ; add boot signature at the end of bootloader
+prep_i13:
+  xor dx, dx
+  div word [bpSectorsPerTrack]
+
+  push dx
+
+  xor dx, dx
+  div word [bpHeads]
+
+  mov ch, al
+  mov dh, dl
+  mov dl, byte [bpDriveNo]
+
+  pop ax
+  mov cl, al
+
+  mov al, 1
+  mov ah, 2
+
+  ret
+
+TIMES 510 - ($ - $$) db 0
+DW 0xAA55
 
 buffer:

kernel.asm

@@ -1,3 +1,17 @@
+  bpBytesPerSector equ 0x0b
+  bpSectorsPerCluster equ 0x0d
+  bpReservedSectors equ 0x0e
+  bpNoFATs equ 0x10
+  bpRootDirEntries equ 0x11
+  bpLVSectors equ 0x13
+  bpMediumID equ 0x15
+  bpSectorsPerFat equ 0x16
+  bpSectorsPerTrack equ 0x18
+  bpHeads equ 0x1a
+  bpHiddenSectors equ 0x1c
+  bpDriveNo equ 0x24
+  bpSignature equ 0x26
+
 [BITS 16]
   ; Update the IVT with our interrupt handler
   mov bx, 0
@@ -8,40 +22,59 @@
   mov ax, 2000h
   mov ds, ax
 
-  pop ax
+  mov ax, 7c0h
-  mov byte [bootdisk], al
+  mov fs, ax
 
   ; Set up kernel specifics
 
   ; Root Directory Entries will be at 2000h:2000h
   ; FAT1 will be at 2000h:4000h
 
+  ; Load sectors RDE to 2000h:2000h
   mov ax, 2000h
   mov es, ax
   mov bx, 2000h
-  mov ah, 2
-  mov al, 16
-  mov cl, 20
-  mov ch, 0
-  mov dh, 0
-  mov dl, [bootdisk]
 
-  ; Load sectors 20 through 36 to 2000h:2000h
+  ; Calculate position of RDE
+  mov ax, word [fs:bpSectorsPerFat]
+  movzx cx, byte [fs:bpNoFATs]
+  mul cx
+
+  add ax, 2
+
+  call prep_i13
+  mov al, 16
+
   int 13h
 
+  ; Load sectors 2 through 36 to 2000h:4000h
   mov ax, 2000h
   mov es, ax
   mov bx, 4000h
+  mov ax, 2
+  call prep_i13
+  mov ax, word [fs:bpSectorsPerFat]
   mov ah, 2
-  mov al, 9
-  mov cl, 2
-  mov ch, 0
-  mov dh, 0
-  mov dl, [bootdisk]
   
-  ; Load sectors 2 through 11 to 2000h:4000h
+  
   int 13h
 
+  ; Calculate file offset
+  mov ax, word [fs:bpSectorsPerFat]
+  movzx cx, byte [fs:bpNoFATs]
+  mul cx
+
+  push ax
+  mov ax, word [fs:bpRootDirEntries]
+  mov cx, 16
+  div cx
+
+  mov bx, ax
+  pop ax
+  add ax, bx
+
+  mov word [fileoffset], ax
+
   push msg
   call print
 
@@ -165,8 +198,6 @@ data:
 
   newline db 13, 10, 0
 
-  bootdisk db 0
-
   program_two db "HELLO   BIN"
 
 
@@ -300,6 +331,9 @@ load_file:
   mov bp, sp
 
   push ds
+  push fs
+  mov ax, 7c0h
+  mov fs, ax
   mov ax, 2000h
   mov ds, ax
 
@@ -308,16 +342,9 @@ load_file:
 
 load_file_loop:
   mov bx, word [bp + 6]
-
+  movzx ax, byte [bp + 8]
-  mov al, byte [bp + 8]
+  add ax, word [fileoffset]
-  add al, 34
+  call prep_i13
-
-  mov cl, al
-  mov al, 1
-  mov ah, 2
-  mov ch, 0
-  mov dh, 0
-  mov dl, byte [bootdisk]
 
   int 13h
 
@@ -334,6 +361,7 @@ load_file_loop:
   jmp load_file_loop
 
 load_file_loaded:
+  pop fs
   pop ds
   pop bp
   ret 6
@@ -369,7 +397,6 @@ decode_filename_loop:
 decode_filename_stage2:
   mov bx, 8
   mov cx, 3
-;  add si, 1
 
 decode_filename_stage2_loop:
 
@@ -419,3 +446,26 @@ _ihdlr_4:
 _ihdlr_5:
 _ihdlr_fin:
   iret
+
+prep_i13:
+  xor dx, dx
+  div word [fs:bpSectorsPerTrack]
+
+  push dx
+
+  xor dx, dx
+  div word [fs:bpHeads]
+
+  mov ch, al
+  mov dh, dl
+  mov dl, byte [fs:bpDriveNo]
+
+  pop ax
+  mov cl, al
+
+  mov al, 1
+  mov ah, 2
+
+  ret
+
+fileoffset dw 0