gistfile1.asm

mainloop:

    // Wait for VBL to process commands
	sbrs	VBL,2		// Bit 2 of VBL register is our rendering window
	rjmp	mainloop

	// Poll the FIFO to see if there's a command pending
	in		accum,PINB
	sbrs	accum,fifoReady
	rjmp	mainloop

	// Set up Port D as an input
	ldi		accum,0x00
	out		DDRD,accum
	ldi		accum,0xff
	clr		accum
	out		PORTD,accum
	nop						// Make sure PORTD has changed direction

	// Read from the FIFO
	sbi		PORTB,vramOE	// Kick VRAM off the bus so we can read the FIFO
	cbi		PORTB,fifoRead
	nop						// Allow for FIFO data setup time
	sbi		PORTB,fifoRead
	in		regS,PIND
	cbi		PORTB,vramOE	// Give bus back to VRAM

	// See if this is a new command, or completing a previous two-byte packet
	lds		accum,CMDBUFFER
	cpi		accum,0
	brne	mainHaveParamByte
	sts		CMDBUFFER,regS

	rjmp	mainloop

mainHaveParamByte:
	sts		CMDPARAM,regS
	lds		regT,CMDBUFFER
	clr		accum
	sts		CMDBUFFER,accum

	cpi		regT,NUM_COMMANDS
	brge	mainloop		// Illegal command, ignore it

	// Bounce off a jump table based on command value
	lsl		regT
	ldi		XL,lo8(pm(renderJumpTable))
	ldi		XH,hi8(pm(renderJumpTable))
	clr		accum
	add		XL,regT
	adc		XH,accum

	push	XL		// Using ret, trick AVR into jumping to an indirect address
	push	XH
	ret				// Note that we're not using ijmp, because we can't use Z

renderJumpTable:
	nop
	rjmp	mainloop
	rcall	renderFillScreen
	rjmp	mainloop
	rcall	renderPlotChar
	rjmp	mainloop
	rcall	renderPlotStr
	rjmp	mainloop
	rcall	renderFontColor
	rjmp	mainloop