blockdev: move the new seek mode logic into the seek command

It wasn't a good idea to complicate all blockdev impls with complicated
seeks. Let's just stay in absolute mode for now.

parts/blockdev.asm

@@ -132,6 +132,33 @@ blkPutC:
 ; 4 : Move to the beginning
 ; Set position of selected device to the value specified in HL
 blkSeek:
+	push	de
+	cp	1
+	jr	z, .forward
+	cp	2
+	jr	z, .backward
+	cp	3
+	jr	z, .beginning
+	cp	4
+	jr	z, .end
+	; all other modes are considered absolute
+	jr	.seek		; for absolute mode, HL is already correct
+.forward:
+	ex	hl, de		; DE has our offset
+	call	blkTell		; HL has our curpos
+	add	hl, de
+	jr	nc, .seek	; no carry? alright!
+	; we have carry? out of bounds, set to maximum
+.backward:
+	; TODO - subtraction are more complicated...
+	jr	.seek
+.beginning:
+	ld	hl, 0
+	jr	.seek
+.end:
+	ld	hl, 0xffff
+.seek:
+	pop	de
 	ld	iyl, 4
 	jr	_blkCall
 

parts/blockdev_cmds.asm

@@ -31,7 +31,6 @@ blkSeekCmd:
 	ld	h, a
 	pop	af	; bring mode back
 	call	blkSeek
-	ld	hl, 42
 	call	blkTell
 	ld	a, h
 	call	printHex

parts/mmap.asm

@@ -57,32 +57,6 @@ mmapPutC:
 	ret
 
 mmapSeek:
-	cp	1
-	jr	z, .forward
-	cp	2
-	jr	z, .backward
-	cp	3
-	jr	z, .beginning
-	cp	4
-	jr	z, .end
-	; all other modes are considered absolute
-	jr	.set		; for absolute mode, HL is already correct
-.forward:
-	ld	de, (MMAP_PTR)
-	add	hl, de
-	jr	nc, .set
-	; we have carry? out of bounds, set to maximum
-	ld	hl, 0xffff
-	jr	.set
-.backward:
-	; TODO - subtraction are more complicated...
-	jr	.set
-.beginning:
-	ld	hl, 0
-	jr	.set
-.end:
-	ld	hl, 0xffff-MMAP_START
-.set:
 	ld	(MMAP_PTR), hl
 	ret