drive i/o: improvements regarding reset and stack usage

- read and write now run in the same function, less branching
- implemented on-demand controller reset
- set up a function stub for int 0x13 for later stack switching

kernel/drive.asm

@@ -17,7 +17,9 @@ biosnum:	resb 1
 		; absolute, ignores partition
 dskseek:	resd 1
 
-		; bit 0 - dirty flag for dskbuf
+		; bit 0 (1) - dirty flag for dskbuf
+		; bit 1 (2) - controller configured
+		; bit 2 (4) - EBIOS supported
 dskflag:	resb 1
 
 		; disk buffer for I/O operations
@@ -42,6 +44,13 @@ dinit:		; copy previously set DPT to our data area
 		mov word [4*0x1E+2], ds
 		ret
 
+		; int 13 stub
+		; place reserved for stack switches
+int13:		;int 3
+		int 0x13
+		;int 3
+		ret
+
 		; restore DPT to default values if possible
 		; IN	dl	bios drive number
 rstdpt:		mov ah, 8
@@ -109,35 +118,6 @@ logdrv:		; dont do anything if drive already selected
 		mov [bpb+BPBHS+2], ax
 .ret:		ret
 
-calchs:		; put sectors per cylinder into bx
-		mov ax, [bpb+BPBSPT]
-		mul word [bpb+BPBNOS]
-		mov bx, ax
-		; put linear sector num into dx:ax
-		mov ax, [dskseek]
-		mov dx, [dskseek+2]
-		; dx:ax = linear count, bx = sectors / cylinder
-		div bx
-		xchg ax, dx
-		; dx = cylinder, ax = head * spt + sector
-		mov bl, [bpb+BPBSPT]
-		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
@@ -175,10 +155,66 @@ read:		push ax
 		mov [dskseek], ax
 		mov [dskseek+2], dx
 		; do the actual read
-		; TODO: error handling & retries
-		call calchs
-		mov ax, 0x0201
-		int 0x13
+
+		; set the operation code and back it up to the stack
+_read:		mov ch, 2
+		db 0x3D ; cmp ax, imm16: causes next instr to be skipped
+_write:		mov ch, 3
+		mov cl, 1 ; read len
+		; TODO: do ebios i/o
+		; check if we can skip controller reset
+		test byte [dskflag], 2
+		jnz .l01
+		; do controller reset
+		mov dl, [biosnum]
+		mov ah, 0
+		call int13
+		or byte [dskflag], 2
+.l01:		; put sectors per cylinder into bx
+		mov ax, [bpb+BPBSPT]
+		mul word [bpb+BPBNOS]
+		mov bx, ax
+		; put linear sector num into dx:ax
+		mov ax, [dskseek]
+		mov dx, [dskseek+2]
+		; dx:ax = linear count, bx = sectors / cylinder
+		div bx
+		xchg ax, dx
+		; dx = cylinder, ax = head * spt + sector
+		mov bl, [bpb+BPBSPT]
+		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
+		; shuffle values around for bios
+		xchg ax, cx
+		xchg cx, dx
+		xchg dh, dl
+		mov dl, [biosnum]
+		mov bx, dskbuf
+		; ah: subfunction selected via cx previously
+		; al: 1 = reading 1 sector
+		; cx: sector and cylinder number
+		; dh: head number
+		; dl: drive number
+		; bx: offset to disk buffer
+		call int13
+		jc .err
+		; clear dirty flag on success
+		and byte [dskflag], 0xFE
+		clc
+		ret
+.err:		; assume controller is misconfigured
+		and byte [dskflag], 0xFD
+		; exit with carry flag set
+		stc
 		ret
 
 		; mark dskbuf as containing unwritten changes
@@ -188,11 +224,6 @@ dirty:		or byte [dskflag], 1 ; dirty
 		; flush buffer if dirty
 flush:		test byte [dskflag], 1
 		jz .ret
-		; do the write
 		; TODO: error handling & retries
-		call calchs
-		mov ax, 0x0301
-		int 0x13
-		; remove dirty flag
-		and byte [dskflag], 0xFE
+		jmp _write
 .ret:		ret