From bd3b27362dd2a2cd3221cf4a7f525d6dec66d50a Mon Sep 17 00:00:00 2001
From: Nero <41307858+nero@users.noreply.github.com>
Date: Thu, 17 Dec 2020 00:10:02 +0000
Subject: [PATCH] skip reads when sector number already matches

---
 kernel/drive.asm | 29 +++++++++++++++++------------
 1 file changed, 17 insertions(+), 12 deletions(-)

diff --git a/kernel/drive.asm b/kernel/drive.asm
index 35d1926..7875c9f 100644
--- a/kernel/drive.asm
+++ b/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