From e096745c1382ae71e864821525414e4fb6cd3e97 Mon Sep 17 00:00:00 2001
From: Nero <41307858+nero@users.noreply.github.com>
Date: Tue, 15 Dec 2020 23:29:35 +0000
Subject: [PATCH] Implement map-like sector access for floppy drives

---
 boot/kernel.asm  |  13 +++-
 kernel/drive.asm | 183 ++++++++++++++++++++++++++++++-----------------
 kernel/fcb.asm   |   2 +-
 3 files changed, 131 insertions(+), 67 deletions(-)

diff --git a/boot/kernel.asm b/boot/kernel.asm
index 74f359d..38afa3b 100644
--- a/boot/kernel.asm
+++ b/boot/kernel.asm
@@ -31,8 +31,17 @@ print_banner:	mov si, banner
 
 		call dskrst
 
-		mov bx, testfcb
-		call fcbfst
+		mov dl, [defdrv]
+		call logdrv
+
+		xor ax, ax
+		xor dx, dx
+		call maprd
+
+		inc word [dskbuf+0x82]
+
+		call dirty
+		call flush
 
 hlt:		hlt
 		jmp hlt
diff --git a/kernel/drive.asm b/kernel/drive.asm
index 17135f1..35d1926 100644
--- a/kernel/drive.asm
+++ b/kernel/drive.asm
@@ -1,10 +1,25 @@
+; code for disk i/o
+; from outside, only the following methods are meant to be called:
+; - logdrv: select drive DL to operate on
+; - map*: map sector DX:AX into dskbuf
+; - dirty: inform that dskbuf has been written to
+; access is done by accessing dskbuf directly
+
 section .bss
 
 		; drive actually selected for I/O
 dsknum:		resb 1
+
+		; bios drive we end up using
+biosnum:	resb 1
+
 		; current sector number
+		; absolute, ignores partition
 dskseek:	resd 1
 
+		; bit 0 - dirty flag for dskbuf
+dskflag:	resb 1
+
 		; disk buffer for I/O operations
 dskbuf:		resb 512
 
@@ -13,69 +28,6 @@ bpb:		resb BPBSIZ4
 
 section .text
 
-ret:		ret
-
-		; Convert drive number into DL for int 13h
-		; A=0, B=1, C=0x80
-		; IN	dl	dos drive number
-		; OUT	dl	bios drive number
-getdl:		cmp dl, 2
-		jc ret
-		mov dl, 0x80
-		ret
-
-		; Ask BIOS for drive parameters
-		; IN	dl	dos drive number
-		; OUT	CX,DH	chs data
-		;	ES:DI	DPT if floppy
-qrychs:		mov ah, 8
-		les di, [0x1E*4]
-		int 0x13
-		jc .defs
-		test cl, cl
-		jz .defs
-		ret
-
-.defs:		mov cx, 0x2709 ; max track = 39, max sec = 9
-		mov dh, 1 ; 2 heads
-		ret
-
-		; select drive
-		; IN	dl	drive number
-dsksel:		cmp dl, byte [dsknum]
-		je ret
-
-		call dskrst
-
-		call getdl
-		push dx
-		call qrychs
-
-		; store CHS data in BPB
-		xor ax, ax
-		mov al, cl
-		mov [bpb+BPBSPT], ax
-		mov al, dh
-		inc ax
-		mov [bpb+BPBNOS], ax
-
-		; get dpt from bios if not hdd
-		pop dx
-		test dl, 0x80
-		jnz .nodpt
-		push dx
-		mov dx, dpt
-		mov bx, di
-		mov cx, DPTSIZE
-		call lod
-		mov ax, [bpb+BPBSPT]
-		mov [dpt+DPTSPT], al
-		pop dx
-.nodpt:
-
-
-		ret
-
 dskrst:		call flush
 		mov ax, 0xFFFF
 		mov [dsknum], al
@@ -83,5 +35,108 @@ dskrst:		call flush
 		mov [dskseek+2], ax
 		ret
 
+		; log in drive
+		; currently only supports 18 sectors 2 heads floppies
+		; IN	dl	drive number
+logdrv:		mov [biosnum], dl
+		xor ax, ax
+		xor dx, dx
+		call read
+
+		; copy bios parameter block
+		lea si, [dskbuf+BPBOFF]
+		mov di, bpb
+		mov cx, BPBSIZ4
+		rep movsb
+
+		; make sure partition offset is force zero
+		xor ax, ax
+		mov [bpb+BPBHS], ax
+		mov [bpb+BPBHS+2], ax
+
+		ret
+
+calchs:		mov bx, (18*2)
+		; dx:ax = linear count, bx = sectors / cylinder
+		div bx
+		xchg ax, dx
+		; dx = cylinder, ax = head * spt + sector
+		mov bl, 18
+		div byte bl
+		; dx = cylinder, al = head, ah = sector
+		xchg dl, dh
+		ror dl, 1
+		ror dl, 1
+		or dl, ah
+		inc dx
+		; al: head number
+		; dh bit 0-7: cylinder bits 0-7
+		; dl bit 0-5: sector bits 0-5
+		; dl bit 6-7: cylinder bits 8-9
+		mov cx, dx
+		mov dh, al
+		mov dl, [biosnum]
+		mov bx, dskbuf
+		ret
+
+mapclu:		; counting from begin of cluster area
+		mov bx, [bpb+BPBRDE]
+		mov cl, 4
+		shr bx, cl ; 32 bytes per entry
+		add ax, bx
+		adc dx, 0
+maprd:		; counting from beginning of dir
+		; add fat table sizes
+		xor ch, ch
+		mov cl, byte [bpb+BPBFN]
+.loop:		add ax, [bpb+BPBFS]
+		adc dx, 0
+		loop .loop
+mapfat:		; counting from beginning of FAT
+		; add reserved sector count
+		add ax, [bpb+BPBRSC]
+		adc dx, 0
+map:		; count from partition start
+		;add ax, [BPBHS]
+		;add dx, [BPBHS+2]
+		; absolute sector count
+mapabs:		call read
+		ret
+
+		; hard read, absolute disk offset
+		; TODO: retries on error
+		; IN    dx:ax
+read:		; flush previous data if necessary
+		push ax
+		push dx
+		call flush
+		pop dx
+		pop ax
+		; store the sector number
+		mov [dskseek], ax
+		mov [dskseek+2], dx
+		; do the actual read
+		; TODO: error handling & retries
+		call calchs
+		mov ax, 0x0201
+		int 0x13
+		ret
+
+		; mark dskbuf as containing unwritten changes
+dirty:		or byte [dskflag], 1 ; dirty
+		ret
+
 		; flush buffer if dirty
-flush:		ret
+flush:		test byte [dskflag], 1
+		jz .ret
+		; load sector number
+		mov ax, [dskseek]
+		mov dx, [dskseek+2]
+		; do the write
+		; TODO: error handling & retries
+		call calchs
+		mov ax, 0x0301
+		int 0x13
+		; remove dirty flag
+		and byte [dskflag], 0xFE
+.ret:		ret
diff --git a/kernel/fcb.asm b/kernel/fcb.asm
index aba293f..2e10f7b 100644
--- a/kernel/fcb.asm
+++ b/kernel/fcb.asm
@@ -17,7 +17,7 @@ setdrv:		mov dl, [es:bx+fcb.drive] ; A=1
 		mov [es:bx+fcb.drive], dl ; A=1
 		; now set as current i/o drive
 .nofix:		dec dl ; A=0 for dsksel
-		jmp dsksel
+		jmp logdrv
 
 		; FCB open
 fcbopn:		; call fcbfst