6502.org

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	.ORG $200
Hello:	.ASCII "Hello!", $00
	.START .Main

.Main	LDY #$00	; String indexer
.getch	LDX Hello, Y	; TODO: String address
	BNE .prtch	; If this is a valid character print it to the screen
	BEQ .fin	; Null terminator? End this now
	
.prtch	TXA		; Transfer X to the accumulator
	STA $E001	; Write the character stored in the accumulator to the screen
	INY		; Point to the next character
	JMP .getch	; Grab the next character
.fin	BRK		; Ends execution?

Code: Select all

	.ORG $200
Hello:	.ASCII "Hello!", $00
	.START .Main

.Main	LDY #$00	; String indexer
.getch	LDA Hello, Y	; TODO: String address
	BEQ .fin	; Null terminator? End this now.  Otherwise . . .
	
.prtch	STA $E001    ; Write the character to the screen
	INY		; Point to the next character
	JMP .getch	; Grab the next character
.fin	BRK		; Ends execution by invoking the system debugger.

Code: Select all

		.ORG $00
DisplayWidth:	.DB $28
DisplayHeight:	.DB $0B
DisplayBaseAddr:.DW $200
DisplayEndAddr:	.DW $3B8
DisplaySize:	.DW $1B8
DisplayPos:	.DW $00		; Counter for screen display ( 16 bits )
Test:		.ASCII "This is a very long string used to test the emulated display memory, will it work? Who knows....", $00
		.START Main
		
; Linear display memory emulator
; Emulated video memory starts at $200 and extends to $3B8
	
; HACKHACKHACK
; Testing purposes only
Main	JSR EmuDisplayClear
	LDY #$00
.NextCh	LDA Test, Y
	STA (DisplayBaseAddr), Y
	BEQ .Fin
	INY
	JMP .NextCh
.Fin
	LDA #$41
	STA $305
	NOP
	NOP
	JSR EmuDisplayCopy
	BRK
	
; Clears out the emulated video memory
; ======================================
EmuDisplayClear						; Clear the first page of VRAM
			LDA #$00			; Load 0 into the accumulator
			LDY #$00			; Load 0 into the Y register

.ClearWriteToVRAM1	STA (DisplayBaseAddr), Y	; Use DisplayBaseAddr as a pointer and store the accumulator into VRAM
			INY				; Next byte
			BEQ .ClearRemainingVRAM		; Did we roll over the Y register? If so, clear the remaining VRAM
			JMP .ClearWriteToVRAM1		; Finish clearing VRAM
			
.ClearRemainingVRAM	LDA DisplayBaseAddr + 1		; Load the high byte of the VRAM address into the accumulator
			ADC #$1				; Add one so we point to the next page
			STA DisplayBaseAddr + 1		; Write back the modified pointer
			LDA #$00			; Reset the accumulator to 0
			
.ClearWriteToVRAM2	STA (DisplayBaseAddr), Y	; Write the accumulator into VRAM
			CPY DisplayEndAddr		; Have we reached the end of VRAM?
			BEQ .ClearCleanup		; If so, clean up what we changed
			INY				; Next byte
			JMP .ClearWriteToVRAM2		; Finish clearing VRAM
			
.ClearCleanup		LDA DisplayBaseAddr + 1		; Load the high byte of the VRAM pointer
			SBC #$1				; Subtract one so we go back to the page it starts on
			STA DisplayBaseAddr + 1		; Write back the pointer
			RTS				; Return to caller

; Writes the emulated video memory to the simulator's console output address
; =============================================================================
EmuDisplayCopy						; Write the first page of VRAM to the simulator console
			LDA #$00
			LDY #$00
			
.CopyCopyVRAMPage1	LDA (DisplayBaseAddr), Y
			INY
			BEQ .CopyRemainingVRAM
			STA $E001
			JMP .CopyCopyVRAMPage1

; BUGBUGBUG
; LDA DisplayBaseAddr + 1 Loads 02 into the accumulator, yet
; ADC #$1 causes the accumulator to read 4 instead of 3
.CopyRemainingVRAM	LDA DisplayBaseAddr + 1		; Load our VRAM pointer's high byte into the accumulator
			ADC #$1				; Add one to set the next page
			STA DisplayBaseAddr + 1		; Save the modified pointer back into ram
			
.CopyVRAMPage2		LDA (DisplayBaseAddr), Y
			INY
			BEQ .CopyCleanup
			STA $E001
			JMP .CopyVRAMPage2
			
.CopyCleanup		LDA DisplayBaseAddr + 1
			SBC #$1
			STA DisplayBaseAddr + 1
			RTS