From: Duje Mihanović <duje.mihanovic@skole.hr>
Date: Sat, 30 Apr 2022 20:23:40 +0000 (+0200)
Subject: Separate loading clusters and cluster chains
X-Git-Tag: 0.1.0~9
X-Git-Url: http://git.dujemihanovic.xyz/%22http:/www.sics.se/static/html/index.html?a=commitdiff_plain;h=c4408dfedc5d7b3a4e6b584c37c58e064265ef2d;p=nameless-os.git

Separate loading clusters and cluster chains

This will be crucial for stage 3 loading the kernel, because we will need to
relocate each cluster to 0x100000 (or somewhere after that) manually, and we
can't just do that in the driver as it's also used by the VBR which does not run
in unreal mode.
---

diff --git a/boot/x86/fat32.s b/boot/x86/fat32.s
index caed036..75e7904 100644
--- a/boot/x86/fat32.s
+++ b/boot/x86/fat32.s
@@ -10,11 +10,24 @@
 ; NOTE: Cluster chain might be the root directory or a file.
 read_cluster_chain:
 	push eax
+	push es
+	push di
+	.loop:
+		call read_cluster
+		cmp eax, 0xffffff7
+		jl .loop
+	pop di
+	pop es
+	pop eax
+	ret
+
+; eax - cluster number
+; es:di - where to load the cluster (incremented automatically)
+; RETURN: eax - next cluster number
+read_cluster:
 	push ebx
 	push ecx
 	push edx
-	push es
-	push di
 .loop:
 	; get the first sector of the cluster to read
 	push eax
@@ -58,8 +71,6 @@ read_cluster_chain:
 	; reminder for myself: EAX is FAT sector number, (E)BX is offset into FAT sector
 	
 	; load the FAT sector we're looking for
-
-	push cx
 	push di
 	push ebx ; offset
 	push es ; we want to read at 0:1000, not STAGE3_SEGMENT:1000
@@ -76,18 +87,10 @@ read_cluster_chain:
 	pop ebx ; pop FAT offset back into EBX for cmp
 	mov eax, [di+bx]
 	pop di
-	pop cx
-	cmp eax, 0xffffff7
-	jl .loop ; if cluster number is lower than the defective cluster value, we're not done
-		 ; TODO: perhaps check is it equal defective and error out in that case?
-.done:
 	; cleanup and return
-	pop di
-	pop es
 	pop edx
 	pop ecx
 	pop ebx
-	pop eax
 	ret
 
 ; es:di - where to load the sector(s)