From 035b69db10a5ae6f091f7951a86531a6216cc909 Mon Sep 17 00:00:00 2001
From: Ain <41307858+nero@users.noreply.github.com>
Date: Wed, 22 Apr 2020 21:45:40 +0200
Subject: [PATCH] fat.bs: Implement correctly for FAT12

---
 boot/fat.asm | 71 ++++++++++++++++++++++++++++++++++++++++++----------
 1 file changed, 58 insertions(+), 13 deletions(-)

diff --git a/boot/fat.asm b/boot/fat.asm
index 46f093d..cdbe931 100644
--- a/boot/fat.asm
+++ b/boot/fat.asm
@@ -55,8 +55,14 @@ readr:	; add offset to rootdir to DX:AX (FAT12/FAT16 only)
 	push cx
 	xor ch, ch
 	mov cl, byte [bp+bpb_fn]
-fatlp:	add ax, [bp+bpb7_lsf]
+fatlp:
+	%ifdef FAT32
+	add ax, [bp+bpb7_lsf]
 	adc dx, [bp+bpb7_lsf+2]
+	%else
+	add ax, [bp+bpb_sf]
+	adc dx, 0
+	%endif
 	loop fatlp
 	pop cx
 readf:	; add offset to FAT table to DX:AX
@@ -168,30 +174,49 @@ shftl:	clc
 
 	pop bx
 
-	; get lower part of cluster number
+	; get address of cluster within sector
 	pop si
+	%ifdef FAT12
+	push si
+	; times 1.5
+	mov ax, si
+	shr ax, 1
+	add si, ax
+	add si, buf
+	lodsw
+	pop si
+	test si, 1
+	jz .odd
+.even:	mov cl, 4
+	shr ax, cl
+	jmp .done
+.odd:	and ax, 0x0FFF
+.done:	xor dx, dx
+	%endif
+
 	%ifdef FAT16
 	; double
         sal si, 1
+	; make sure its within sector range
+	and si, 0x1FF
+	add si, buf
+	; load word
+	lodsw
+	xor dx, dx
 	%endif
+
 	%ifdef FAT32
 	; multiply with 4
 	sal si, 1
 	sal si, 1
-	%endif
 	; make sure its within sector range
-        and si, 0x1FF
+	and si, 0x1FF
 	add si, buf
-	%ifdef FAT16
-	; load word from FAT
-	lodsw
-	xor dx, dx
-	%endif
-	%ifdef FAT32
-	; load dword from FAT
+	; load dword
 	lodsw
 	mov dx, [si]
 	%endif
+
 	ret
 
 	; reads current cluster into [dest]
@@ -230,9 +255,21 @@ l02e:
 	ret
 
 	; Load root directory
-loadr:	mov ax, 2
+loadr:
+	%ifdef FAT32
+	mov ax, 2
 	xor dx, dx
 	jmp load
+	%else
+	mov ax, [bp+bpb_rde]
+	mov cl, 4
+	shr ax, cl
+	mov cx, ax
+	xor ax, ax
+	xor dx, dx
+	mov bx, 0x07C0
+	jmp readr
+	%endif
 
 	; Load file identified by fsattr
 	; Directory data is expected at 0x07C00
@@ -259,8 +296,16 @@ sloop:	add si, 0x20
 load:	mov bx, 0x07C0
 lloop:	call readc
 	call next
+	%ifdef FAT12
+	cmp word ax, 0x0FF8
+	%endif
+	%ifdef FAT16
+	cmp word ax, 0xFFF8
+	%endif
+	%ifdef FAT32
 	cmp word dx, 0x0FFF
-	jne lloop
+	%endif
+	jc lloop
 	ret
 
 main:	; Set up BP to point to BPB