start stage 2 partition search

boot0.s

@@ -22,6 +22,10 @@
 ; bp - 0x0c: sector stride when loading GPT entries
 ; bp - 0x0e: byte stride when loading GPT entries
 ; bp - 0x10: entries per sector when loding GPT entries
+; bp - 0x12: current entry index
+; bp - 0x14: current entry index within the current sector
+; bp - 0x16: number of sectors loaded
+; bp - 0x18: current LBA
 
 ; BIOS puts our boot sector at 0000:7c00
 org 0x7c00
@@ -42,10 +46,6 @@ bits 16
   mov bp, 0x7c00
   mov sp, bp
 
-  ; Segment for VGA (0xb800 * 16 = 0xb8000)
-  mov ax, 0xb800
-  mov fs, ax
-
   ; Store boot drive number
   xor dh, dh
   push dx
@@ -71,9 +71,9 @@ bits 16
   inc cl
   xor ah, ah ; Zero-extend temp
   xor dx, dx ; div by 16-bit register divides dx:ax, so we zero dx
-  div bx     ; ah = mod (head), al = div (cylinder)
+  div bx     ; dx = mod (head), ax = div (cylinder)
   
-  mov dh, ah ; Head
+  mov dh, dl ; Head
   mov ch, al ; Cylinder
              ; We already have sector in cl
   mov ah, 2  ; Read disk
@@ -86,11 +86,6 @@ bits 16
   ; head = temp % number of heads
   ; cylinder = temp / number of heads
 
-  ; Set VGA mode
-  ; https://mendelson.org/wpdos/videomodes.txt
-  mov ax, 0x0003
-  int 0x10
-
   mov cx, 8
   mov si, .gpt_magic
   mov di, 0x7e00
@@ -99,13 +94,10 @@ bits 16
 
   ; Ensure the 8-byte GPT starting LBA fits in 16 bits
   mov di, 0x7e00 ; The rep increments di so we need to reset it
-  xor bx, bx
-  mov ax, [di + 0x4a]
-  or bx, ax
-  mov ax, [di + 0x4c]
-  or bx, ax
-  mov ax, [di + 0x4e]
-  or bx, ax
+  mov eax, [di + 0x4c]
+  mov bx, [di + 0x4a]
+  or ax, bx
+  or eax, eax
   jnz .panic
   ; Load and spill the GPT starting LBA
   mov ax, [di + 0x48]
@@ -153,6 +145,8 @@ bits 16
 .gpt_sector_stride_loaded:
   push ax
 
+  ; Find the "byte stride", which is the number of bytes we increment by each time we want to load
+  ; the next entry in the same sector.
   cmp ebx, 512
   jb .gpt_find_entries_per_sector
   push word 0 ; Arbitrary byte stride since there's only one entry per sector
@@ -166,13 +160,75 @@ bits 16
   push ax
 .gpt_found_entries_per_sector:
 
+  ; Set up stack variables for our second stage search loop.
+  xor ax, ax
+  push ax             ; Current entry
+  push ax             ; Current entry within the current sector
+  push ax             ; First LBA loaded?
+  mov ax, [bp - 0x08] ; Starting LBA
+  push ax             ; Current LBA
+
+  ; Search for the partition storing our second stage.
+.loop_find_stage2:
+  mov dx, [bp - 0x12]
+  cmp [bp - 0x0a], dx
+  ; Panic if we've run out of partitions and haven't found the second stage yet.
+  jbe .panic
+
+  ; If we haven't loaded any sectors yet, load the first one.
+  cmp word [bp - 0x16], 0   ; Load number of sectors loaded
+  je .load_first_lba
+  ; If there's still more entries in the current sector, skip loading a new sector
+  mov ax, [bp - 0x18]       ; Load current entry index within the current sector
+  cmp [bp - 0x10], ax       ; Compare to entries per sector
+  ja .process_current_entry
+  
+  mov ax, [bp - 0x0c]      ; Load sector stride
+  add word [bp - 0x18], ax ; Increment current LBA by sector stride
+  mov word [bp - 0x14], 0  ; Reset the current entry index within the current sector
+.load_first_lba:
+  mov ax, [bp - 0x18]      ; Load current LBA
+  xor dx, dx
+  div word [bp - 0x04]     ; Divide by sectors per track. dx = mod (sector - 1), ax = div (temp)
+  mov cx, dx
+  inc cx                   ; Put sector into cx (bios will read sector from cl)
+  xor dx, dx
+  div word [bp - 0x06]     ; Divide by number of heads. dx = mod (head), ax = div (cylinder)
+  mov dh, dl
+  mov ch, al
+  mov dl, byte [bp - 0x02] ; Load drive number
+  mov ah, 0x02
+  mov al, 1
+  mov bx, 0x8000
+  int 0x13                 ; Read sector
+  jc .panic
+  inc word [bp - 0x16]     ; Increment number of sectors loaded
+
+.process_current_entry:
+  jmp .debug
+  ; TODO
+  ; TODO: remember to read offset by current entry index within the current sector
+
+  inc word [bp - 0x12]  ; Increment current entry index
+  inc word [bp - 0x14]  ; Increment current entry index within the current sector
+  jmp .loop_find_stage2
+
+.debug:
+  ; Segment for VGA (0xb800 * 16 = 0xb8000)
+  mov ax, 0xb800
+  mov fs, ax
+
+  ; Set VGA mode
+  ; https://mendelson.org/wpdos/videomodes.txt
+  mov ax, 0x0003
+  int 0x10
+
   mov al, [0x7e00]
   mov byte fs:[0x0000], al
-  mov al, [0x7e01]
+
+  mov al, [0x8000]
   mov byte fs:[0x0002], al
-  mov al, [0x7e02]
-  mov byte fs:[0x0004], al
-  
+
   ; mov word fs:[0x0000], 0xc048
   ; mov word fs:[0x0002], 0xc069
   
@@ -183,10 +239,7 @@ bits 16
   int 0x10
   mov ax, 0xb800
   mov ds, ax
-  mov word [0x0000], 0x4f46
-  mov word [0x0002], 0x4f41
-  mov word [0x0004], 0x4f49
-  mov word [0x0006], 0x4f4c
+  mov word [0x0000], 0x4f21
   hlt
 
   ; TODO: enable A20
@@ -209,6 +262,10 @@ bits 16
 .gpt_magic:
   db "EFI PART"
 
+.guid_linux:
+  db 0xaf, 0x3d, 0xc6, 0x0f, 0x83, 0x84, 0x72, 0x47, \
+     0x8e, 0x79, 0x3d, 0x69, 0xd8, 0x47, 0x7d, 0xe4
+
   ; MBR bootstrap field is 440 bytes long
   %if ($ - $$) > 440
   %error "exceeded mbr bootstrap field size"