ti/lcd: Add scrolling through z-offset

kernel/ti/kbd.asm

@@ -147,6 +147,6 @@ kbdGetC:
 	.db	0xfd, 0x0d, '"' ,'W' ,'R', 'M', 'H', 0, 0
 	.db	0xfb, '?', 0, 'V', 'Q', 'L', 'G', 0, 0
 	.db	0xf7, ':', 'Z', 'U', 'P', 'K', 'F', 'C', 0
-	.db	0xef, '_', 'Y', 'T', 'O', 'J', 'E', 'B', 0
+	.db	0xef, ' ', 'Y', 'T', 'O', 'J', 'E', 'B', 0
 	.db	0xdf, 0, 'X', 'S', 'N', 'I', 'D', 'A', KBD_KEY_ALPHA
 	.db	0xbf, 0, 0, 0, 0, 0, KBD_KEY_2ND, 0, 0x7f

kernel/ti/lcd.asm

@@ -2,9 +2,30 @@
 ;
 ; Implement PutC on TI-84+ (for now)'s LCD screen.
 ;
+; The screen is 96x64 pixels. The 64 rows are addressed directly with CMD_ROW
+; but columns are addressed in chunks of 6 or 8 bits (there are two modes).
+;
+; In 6-bit mode, there are 16 visible columns. In 8-bit mode, there are 12.
+;
 ; Note that "X-increment" and "Y-increment" work in the opposite way than what
 ; most people expect. Y moves left and right, X moves up and down.
 ;
+; *** Z-Offset ***
+;
+; This LCD has a "Z-Offset" parameter, allowing to offset rows on the
+; screen however we wish. This is handy because it allows us to scroll more
+; efficiently. Instead of having to copy the LCD ram around at each linefeed
+; (or instead of having to maintain an in-memory buffer), we can use this
+; feature.
+;
+; The Z-Offet goes upwards, with wrapping. For example, if we have an 8 pixels
+; high line at row 0 and if our offset is 8, that line will go up 8 pixels,
+; wrapping itself to the bottom of the screen.
+;
+; The principle is this: The active line is always the bottom one. Therefore,
+; when active row is 0, Z is FNT_HEIGHT+1, when row is 1, Z is (FNT_HEIGHT+1)*2,
+; When row is 8, Z is 0.
+;
 ; *** Requirements ***
 ; fnt/mgm
 ;
@@ -41,11 +62,15 @@ lcdInit:
 	; Clear screen
 	call	lcdClrScr
 
+	; We begin with a Z offset of FNT_HEIGHT+1
+	ld	a, LCD_CMD_ZOFFSET+FNT_HEIGHT+1
+	call	lcdCmd
+
 	; Enable the LCD
 	ld	a, LCD_CMD_ENABLE
 	call	lcdCmd
 
-	; Hack to get LCD to work. According to WikiTI, we're to sure why TIOS
+	; Hack to get LCD to work. According to WikiTI, we're not sure why TIOS
 	; sends these, but it sends it, and it is required to make the LCD
 	; work. So...
 	ld	a, 0x17
@@ -151,15 +176,26 @@ lcdSendGlyph:
 lcdLinefeed:
 	push	af
 	ld	a, (LCD_CURROW)
-	add	a, FNT_HEIGHT+1
+	call	.addFntH
 	ld	(LCD_CURROW), a
+	call	lcdClrLn
+	; Now, lets set Z offset which is CURROW+FNT_HEIGHT+1
+	call	.addFntH
+	add	a, LCD_CMD_ZOFFSET
+	call	lcdCmd
 	xor	a
 	ld	(LCD_CURCOL), a
 	pop	af
 	ret
+.addFntH:
+	add	a, FNT_HEIGHT+1
+	cp	64
+	ret	c		; A < 64? no wrap
+	; we have to wrap around
+	xor	a
+	ret
 
 ; Clears B rows starting at row A
-; The LCD will then be set back at row A, column 0
 ; B is not preserved by this routine
 lcdClrX:
 	push	af