Subject: Re: [stella] A question regarding write only addresses From: Erik Mooney <erik@xxxxxxxxxx> Date: Fri, 20 Jul 2001 00:44:03 -0400 |
On Tue, 20 Jul 1993 20:40:56 -0300, you wrote: >Hi. This is my first post, and I´m not really sure how to make a post... >hope this came up right! > >Also excuse my english, I will try to be clear, but ... Both your post and your English look fine to me :) >Here is the state of the 6507: > > A=02 X=04 Y=18 S=FD PC=1441 N=0 V=0 U=1 B=0 D=0 I=1 Z=0 C=1 > > 1441 E50F SBC $0F [000F]=00 > >Has you can see, when this opcode is executed, the accumulator will be >subtracted with the content of the address $000F. To my current knowledge >this address correspond to PF2 which is write only. Both PCAE and Z26, after >executing this opcode, leave the accumulator with $F3, so, the value >subtracted was $F. All the TIA graphics and sound registers are indeed write-only. When you read from anything in the TIA address space (which is $0 through $7F and any mirrors thereof when lines A7 and A12 are low), you get its read-only registers, which are the collision registers from $0 to $7, the four paddle potentiometer inputs from $8 to $B, and the joystick buttons at $C and $D. Reading from $F is undefined as far as I know. Your guess was correct, I believe. With undefined situations like this in the 2600, almost always the value read will be whatever value was on the bus on the previous cycle. I remember Video Pinball relies on that for some operations involving collision detection, which was why that took so long to emulate properly. Looks like this may be a similar case. >Does this happens too with read addresses like INPT4? If I do LDA INPT4, >will >accum bits 6 to 0 contain the last bus value? Yes. In fact, that is actually the detail that Video Pinball relies on and needs in an emulator, which took everyone a good long while to figure out. - Archives (includes files) at http://www.biglist.com/lists/stella/archives/ Unsub & more at http://www.biglist.com/lists/stella/
Current Thread |
---|
|
<- Previous | Index | Next -> |
---|---|---|
[stella] A question regarding write, Nicolás Olhaberry | Thread | [stella] Timing blues..., Christopher Rydberg |
[stella] A question regarding write, Nicolás Olhaberry | Date | Re: [stella] OT: Virus at stellalis, Phillip Powers |
Month |