[stella] Reading T1024T?

[seagtgruff at aol.com]

Last night I figured out that there are 1024 different memory addresses that can be used for TIMINT. Besides T1024T ($0297), TIM8T ($0295) would also work as a mirror. I guess using T1024T to read the timer interrupt flag is no more erroneous than reading RESP0 ($0010) to check CXM0P ($0000), since $0010 is a mirror of $0000 with regard to TIA read registers. If we don't have the original source code for "Astroblast," then I guess we don't know whether the programmer defined $0297 as T1024T or TIMINT. After all, we define $0000 as VSYNC (write) and CXM0P (read). But I figured that when there are many different possible mirrors that could be used, the one with the lowest address (i.e., using 0s in all of the ignored bits) was the one that's normally used-- unless there's a specific reason to use a higher address, such as relocating the TIA registers to $0040 when using "3F" TigerVision bankswitching.



Michael


-----Original Message-----
From: Dennis Debro <dennis at debrofamily.com>
To: Atari 2600 programming list <stella at atari2600.org>
Sent: Mon, 7 Jan 2008 9:03 am
Subject: Re: [stella] Reading T1024T?




Hi Michael,

> I figured it out. It's either a "mistake" or a "clever trick," depending
> on your point of view.
> "LDA T1024T" apparently has the same effect as "LDA TIMINT." It's actually
> reading TIMINT-- or mirror of TIMINT-- rather than T1024T.

Thanks, Michael! I see I was reading the datasheet wrong.

Based on this I agree with you that this was a mistake rather than a
trick. There's nothing tricky about it IMO.

Take care,
Dennis


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