Disk read refactor

src/bios/disk.asm

@@ -1,21 +1,42 @@
+;disk_read:
+;	push dx
+;	mov ah, BIOS_DISK_READ 
+;	mov al, dh ; sector count 
+;	mov cl, 0x02 ; second sector (after bootsection)
+;
+;	mov ch, 0x00 ; read from first cylinder
+;	mov dh, 0x00 ; head
+;
+;	; data pointer: es:bx (standard)
+;	int BIOS_DISK_INT ; do the interrupt
+;	jc read_error ; if flag is set then jump to error
+;
+;	pop dx
+;	cmp al, dh
+;	jne sector_error
+;	
+;	ret 
+
 disk_read:
-	push dx
-	mov ah, BIOS_DISK_READ 
-	mov al, dh ; sector count 
-	mov cl, 0x02
+	push dx ; store dx on stack so that we can compare later
+
+	mov ah, BIOS_DISK_READ ; specify function
 
-	mov ch, 0x00 ; read from first cylinder
-	mov dh, 0x00 ; head
+	mov al, dh ; read dh amount of sectors
+	mov ch, 0x00 ; CYLINDER
+	mov dh, 0x00 ; HEAD
+	mov cl, 0x02 ; SECTOR
 
-	; data pointer: es:bx (standard)
-	int BIOS_DISK_INT ; do the interrupt
-	jc read_error ; if flag is set then jump to error
+	int BIOS_DISK_INT ; interrupt
+
+	; Error checks
+	jc read_error ; carry flag set -> error
 
 	pop dx
-	cmp al, dh
+	cmp dh, al ; if dh != al then error
 	jne sector_error
-	
-	ret 
+
+	ret
 
 sector_error:
 	mov bx, sector_error_string
@@ -31,5 +52,5 @@ read_error:
 disk_loop:
 	jmp $
 
-read_error_string:		db "Disk read error", ASCII_END
-sector_error_string:	db "Invalid number of sectors read", ASCII_END
+read_error_string:		db "Disk read error!", ASCII_END
+sector_error_string:	db "Invalid number of sectors read!", ASCII_END