blockdev: reorganize routine pointers

Also, add dummy seek reoutine.

parts/blockdev.asm

@@ -17,15 +17,13 @@ BLOCKDEV_ERR_OUT_OF_BOUNDS	.equ	0x03
 
 ; *** VARIABLES ***
 ; A memory pointer to a device table. A device table is a list of addresses
-; pointing to GetC and PutC routines.
+; pointing to GetC, PutC and Seek routines.
 BLOCKDEV_TBL		.equ	BLOCKDEV_RAMSTART
-; Index of the current blockdev selection
-BLOCKDEV_SELIDX		.equ	BLOCKDEV_TBL+(BLOCKDEV_COUNT*4)
-; Address of the current GetC routine
-BLOCKDEV_GETC		.equ	BLOCKDEV_SELIDX+1
-; Address of the current PutC routine
-BLOCKDEV_PUTC		.equ	BLOCKDEV_GETC+2
-BLOCKDEV_RAMEND		.equ	BLOCKDEV_PUTC+2
+; Pointer to the selected block device. A block device is a 6 bytes block of
+; memory with pointers to GetC, PutC and Seek routines, in that order. 0 means
+; unsupported.
+BLOCKDEV_SEL		.equ	BLOCKDEV_TBL+(BLOCKDEV_COUNT*2)
+BLOCKDEV_RAMEND		.equ	BLOCKDEV_SEL+2
 
 ; *** CODE ***
 ; set DE to point to the table entry at index A.
@@ -35,41 +33,29 @@ blkFind:
 	ret	z	; index is zero? don't loop
 	push	bc
 	ld	b, a
-	push	af
-	ld	a, 4
 .loop:
-	call	addDE
+	inc	de
+	inc	de
 	djnz	.loop
-	pop	af
 	pop	bc
 	ret
 
-; Set the GetC pointer of device id A to the value in HL
-blkSetGetC:
-	call	blkFind
-	call	writeHLinDE
-	ret
-
-; Set the GetC pointer of device id A to the value in HL
-blkSetPutC:
+; Set the pointer of device id A to the value in HL
+blkSet:
 	call	blkFind
-	inc	de
-	inc	de
 	call	writeHLinDE
 	ret
 
 ; Select block index specified in A
 blkSel:
+	push	de
+	push	hl
 	call	blkFind
-	ld	(BLOCKDEV_SELIDX), a
+	ld	hl, BLOCKDEV_SEL
 	ex	hl, de
-	; now, HL points to the table entry
-	ld	de, BLOCKDEV_GETC
-	ldi	; copy (HL) into (BLOCKDEV_GETC)
-	ldi	; .. and into +1
-	ld	de, BLOCKDEV_PUTC
-	ldi	; same thing for (BLOCKDEV_PUTC)
 	ldi
+	pop	hl
+	pop	de
 	ret
 
 blkBselCmd:
@@ -85,12 +71,23 @@ blkBsel:
 	ld	a, BLOCKDEV_ERR_OUT_OF_BOUNDS
 	ret
 
-; Reads one character from blockdev ID specified at A and returns its value
-; in A. Always returns a character and waits until read if it has to.
-blkGetC:
+; In those routines below, IY is destroyed (we don't push it to the stack). We
+; seldom use it anyways...
+
+; call routine in BLOCKDEV_SEL with offset IYL.
+_blkCall:
 	push	ix
 	push	de
-	ld	de, (BLOCKDEV_GETC)
+	ld	de, (BLOCKDEV_SEL)
+	; DE now points to the *address table*, not the routine addresses
+	; themselves. One layer of indirection left.
+	; slide by offset
+	push	af
+	ld	a, iyl
+	call	addDE	; slide by offset
+	pop	af
+	call	intoDE
+	; Alright, now de points to what we want to call
 	ld	ixh, d
 	ld	ixl, e
 	pop	de
@@ -98,13 +95,17 @@ blkGetC:
 	pop	ix
 	ret
 
+; Reads one character from blockdev ID specified at A and returns its value
+; in A. Always returns a character and waits until read if it has to.
+blkGetC:
+	ld	iyl, 0
+	jr	_blkCall
+
 blkPutC:
-	push	ix
-	push	de
-	ld	de, (BLOCKDEV_PUTC)
-	ld	ixh, d
-	ld	ixl, e
-	pop	de
-	call	callIX
-	pop	ix
-	ret
+	ld	iyl, 2
+	jr	_blkCall
+
+blkSeek:
+	ld	iyl, 4
+	jr	_blkCall
+