skip reads when sector number already matches

kernel/drive.asm

@@ -38,10 +38,15 @@ dskrst:		call flush
 		; log in drive
 		; currently only supports 18 sectors 2 heads floppies
 		; IN	dl	drive number
-logdrv:		mov [biosnum], dl
+logdrv:		push dx
+		call dskrst
+		pop dx
+
+		mov [biosnum], dl
+
 		xor ax, ax
 		xor dx, dx
-		call read
+		call mapabs
 
 		; copy bios parameter block
 		lea si, [dskbuf+BPBOFF]
@@ -49,7 +54,7 @@ logdrv:		mov [biosnum], dl
 		mov cx, BPBSIZ4
 		rep movsb
 
-		; make sure partition offset is force zero
+		; make sure partition offset is forced zero
 		xor ax, ax
 		mov [bpb+BPBHS], ax
 		mov [bpb+BPBHS+2], ax
@@ -97,17 +102,17 @@ mapfat:		; counting from beginning of FAT
 		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
+		add ax, [bpb+BPBHS]
+		add dx, [bpb+BPBHS+2]
+mapabs:		; absolute sector count
+		; skip doing a read if sector number matches
+		cmp ax, [dskseek]
+		jne read
+		cmp dx, [dskseek+2]
+		jne read
 		ret
 
-		; hard read, absolute disk offset
-		; TODO: retries on error
-		; IN    dx:ax
-read:		; flush previous data if necessary
-		push ax
+read:		push ax
 		push dx
 		call flush
 		pop dx