X-Git-Url: http://git.dujemihanovic.xyz/posts?a=blobdiff_plain;f=boot%2Fx86%2Fvbr-fat32.s;h=439157f3c4299b6f7bf116f3f519fcbb0d860deb;hb=5138270e466cf2dd21bf0aa6356672fdcc85fee6;hp=061ca527afc3d585558c43b495c72a68536c6476;hpb=dad65146188909b9568ed39b3b3fbc5e88e158c3;p=nameless-os.git

diff --git a/boot/x86/vbr-fat32.s b/boot/x86/vbr-fat32.s
index 061ca52..439157f 100644
--- a/boot/x86/vbr-fat32.s
+++ b/boot/x86/vbr-fat32.s
@@ -2,14 +2,19 @@
 ; This is what's going to be on most USB sticks and HDDs, for now
 
 bits 16
-org 0x7c00
+org 0xfe00
 cpu 686
 
-STAGE3_ADDRESS equ 0x8000
+STAGE3_ADDRESS equ 0x1800
 STAGE3_SEGMENT equ STAGE3_ADDRESS >> 4
 STAGE3_OFFSET equ STAGE3_ADDRESS & 0xf
 
-%include "fat32-structs.s"
+%include "fat32/fat32-structs.s"
+
+%macro print 1
+	mov si, %1
+	call print_str
+%endmacro
 
 _start:
 	jmp short real_start
@@ -20,11 +25,10 @@ times 0x57 db 0 ; skip past BPB
 real_start:
 	sti
 	; no need to set up segments and stack again, because MBR did it for us
-	mov bp, 0x7c00
+	mov bp, 0xfe00
 
 	; we expect the boot drive to be in DL and our partition table entry in DS:SI
 	mov [BOOT_DRIVE], dl
-	mov [part_table_entry], si
 
 	; calculate the 1st sector of the data area
 	call get_1st_data_sec
@@ -37,7 +41,9 @@ real_start:
 	mov eax, BPB_RootClus
 	call read_cluster_chain
 
+	push cx
 	mov cx, 11
+	push si
 .find_stage_3:
 	mov si, STAGE3_NAME
 	cmp byte [es:di], 0 ; we have no more entries to look at
@@ -53,29 +59,33 @@ real_start:
 	add bx, 0x1000
 	mov es, bx
 .stage3_found:
-	sub di, 0xb ; "repe cmpsb" incremented this
+	pop si
+	pop cx
+	add di, 9 ; knowing that cmpsb incremented this by 11, we can also increment it by 9
+		       ; right here so we can use 1 less offset below
 	; stage 3 has been found and ES:DI points to its directory entry
-	mov ax, [es:di+dir_entry.firstclushi] ; load high half of the 1st cluster
+	mov ax, [es:di] ; load high half of the 1st cluster
 	shl eax, 16
-	mov ax, [es:di+dir_entry.firstcluslo] ; load low half of the 1st cluster
+	mov ax, [es:di+(dir_entry.firstcluslo-dir_entry.firstclushi)] ; load low half of the 1st cluster
 	mov bx, STAGE3_SEGMENT
 	mov es, bx
 	mov di, STAGE3_OFFSET
 	call read_cluster_chain ; read stage 3
-	mov ds, bx
-	call STAGE3_SEGMENT:STAGE3_OFFSET ; call stage 3
+	mov dl, [BOOT_DRIVE]
+	call STAGE3_ADDRESS ; call stage 3
 	jmp .halt ; halt in case we return, which should never happen
 
 .stage3_missing:
-	mov si, stage3_missing
-	call print
+	print stage3_missing
+	jmp .halt
+.error:
+	print read_error
 .halt:
-	cli
 	hlt
 	jmp short $-1
 
 ; ds:si - string
-print:
+print_str:
 	pusha
 	mov ah, 0xe
 	xor bh, bh
@@ -93,7 +103,7 @@ print:
 	popa
 	ret
 
-%include "fat32.s"
+%include "fat32/fat32.s"
 
 stage3_missing: db "LOADER.BIN is missing", 0
 STAGE3_NAME: db "LOADER  BIN"