Reading from the LCD - emulators get it wrong

[Goplat]

It's well known that when reading LCD data from port $11, you have to do an extra "dummy read" first before you can start getting the actual data you want. Emulators just return 0 for this dummy read (or 17 on wabbitemu). But on my calc I've found I can get it to return any value I want. It looks like the dummy read is, to the LCD driver, just like any other read - but whenever you read from port $11, it reads from the current coordinates, but returns the value of the previous read.

Here's the program I used for testing. It displays 170 ($AA) on a real TI-83+.

Code: Select all

	di
	b_call($42B5)	; set X-increment mode

	call	top
	ld	a,$aa
	out	($11),a
	call	top
	in	a,($11)	; reads $aa, sends previous read to CPU

	call	top
	ld	a,$ff
	out	($11),a
	call	top
	in	a,($11) ; reads $ff, sends $aa to CPU

	ld	h,0
	ld	l,a
	b_call(_DispHL)
	ei
	ret
top:	call	$000b
	ld	a,$20
	call	$+5
	ld	a,$80
	out	($10),a
	jp	$000b

[Spencer]

I've modified Wabbitemu to return a value according to your findings. Who can verify the other models?

[Dwedit]

Wow, that was the way I designed my emulator's LCD (just save the last value read and read that next time), not only is it the simplest way to emulate the requirement of the garbage read, it also turns out to be the correct way to do it!

[CoBB]

Wow, that was the way I designed my emulator's LCD (just save the last value read and read that next time), not only is it the simplest way to emulate the requirement of the garbage read, it also turns out to be the correct way to do it!

Of course it makes sense, since that makes the simplest circuit in reality too. For instance, only when I wrote my own Z80 core I understood why the byte order for DDCB/FDCB instructions defines the opcode of the actual operation to come after the relative address (just like above, it saves a register in the circuit).

[Dwedit]

Well, I would have thought it would have returned the last byte either read from or written to, not just read from.

[tr1p1ea]

Interesting find Goplat, and welcome to the forums btw .