Merge pull request #65 from agurtovoy/feat/screen

feat: `screen`/`screen$` support

Author: agurtovoy

bin/basic.rom

@@ -85,15 +85,15 @@ EXTScreenRowOffsets	.fill 	128 * 2
 		.section zeropage
 
 ;;
-; Current screen line address pointer.
+; Current screen line address for write operations.
 ;
 ; 16-bit pointer to the screen address marking the start of the current line.
 ; This address is calculated from `EXTMemory` + row offset and is used for
-; fast character positioning and screen operations.
+; fast character positioning in screen write operations.
 ;
 ; \size    2 bytes
 ; \note    Located in zero page for efficient indirect addressing.
-; \see     EXTMemory, EXTRow, EXTColumn, EXTScreenRowOffsets
+; \see     EXTMemory, EXTRow, EXTColumn, EXTScreenRowOffsets, EXTReadAddress
 ;;
 EXTAddress			.fill 	2
 

howto/howto-crossdev-asm.zip

@@ -0,0 +1,45 @@
+.section code
+
+;;
+; Read character from screen at specified coordinates.
+;
+; Retrieves the character stored at the given row and column position on the
+; screen. Uses precomputed row offsets for efficient address calculation and
+; indirect addressing for memory access. No bounds checking is performed on
+; the coordinates; the caller must ensure that the row and column coordinates
+; are within valid ranges.
+;
+; \in A         Row coordinate (0-based).
+; \in Y         Column coordinate (0-based).
+; \out A        Character at the specified screen position.
+; \sideeffects  - Modifies `A` register and `zTemp0`.
+; \see          EXTScreenRowOffsets, EXTMemory, zTemp0
+;;
+Export_EXTScreenAt:
+		phy									; save column coordinate on stack
+
+		; lookup the corresponding row offset
+		asl 	a							; multiply row index by 2 to get byte index
+		tay									; `Y` holds the byte index of the row offset
+
+		lda 	#2 							; select text page
+		sta 	$0001
+
+		; add row offset to the beginning of the text memory address,
+		; store the result in `zTemp0`
+		clc
+		lda 	#<EXTMemory					; `A` = low byte of screen memory
+		adc 	EXTScreenRowOffsets,y		; add the row offset
+		sta 	zTemp0						; store low byte of the line address
+
+		lda 	#>EXTMemory					; `A` = high byte of screen memory
+		adc 	EXTScreenRowOffsets+1,y		; add the row offset
+		sta 	zTemp0+1					; store high byte of the line address
+
+		; read character from screen memory at (row, column)
+		ply									; restore column index into `Y`
+		lda 	(zTemp0),y					; zTemp0 + Y = character address
+
+		rts
+
+		.send code