Build drive selection logic for FCBs

kernel/drive.asm

@@ -1,6 +1,6 @@
 section .bss
 
-		; drive selected for I/O
+		; drive actually selected for I/O
 dsknum:		resb 1
 		; current sector number
 dskseek:	resd 1

kernel/fcb.asm

@@ -1,8 +1,23 @@
-; in FCB required
-; word	offset in rootdir
-;	used for search
-; word	current cluster
-; byte	record num in cluster
+section .bss
+
+		; default drive if field in FCB = 0
+defdrv:		resb 1
+
+section .text
+		; Functions here generally expect the FCB to be at ES:BX
+
+		; Helper function: read the drive number from FCB,
+		; apply default if necessary,
+		; and make it current I/O drive
+setdrv:		mov dl, [es:bx+fcb.drive] ; A=1
+		test dl, dl
+		jnz .nofix
+		mov dl, [defdrv] ; A=0
+		inc dl
+		mov [es:bx+fcb.drive], dl ; A=1
+		; now set as current i/o drive
+.nofix:		dec dl ; A=0 for dsksel
+		jmp dsksel
 
 		; FCB open
 fcbopn:		; call fcbfst
@@ -16,12 +31,10 @@ fcbcls:		; flush disk buffer
 		ret
 
 		; FCB find first
-fcbfst:		; init search state in FCB
-		; jmp to fcbnxt
-		ret
-
+fcbfst:		mov word [es:bx+fcb.dirpos], 0
 		; FCB find next
-fcbnxt:		; load rootdir sector from state
+fcbnxt:		call setdrv
+		; load rootdir sector from state
 		; search until next match
 		; next sec & loop if mismatch
 		; err exit if no further matches
@@ -31,6 +44,7 @@ fcbnxt:		; load rootdir sector from state
 fcbdel:		; call fcbfst
 		; get search state, load root dir from there
 		; mark file as deleted
+		; mark buffer dirty
 		ret
 
 		; FCB read