Subject: [stella] A question regarding write only addresses From: Nicolás Olhaberry <nolh@xxxxxxxxxxxxx> Date: Tue, 20 Jul 1993 20:40:56 -0300 |
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 ... Well, I have been working on an atari emulator for a while (as a hobby, the world doesn´t need another atari emu) and I´m current having a problem with the game "Haunted House" that I like to discuss. I´m using the 4k PAL version, by the way. 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. In my emu, since is subtracting zero, the carry remains set which makes the game to enter into an endless loop, look at this code: 1440 C8 INY 1441 E50F SBC $0F [000F]=00 1443 B0FB BCS $1440 If I change the opcode in the ROM for SBC #$0F (E90F) the game runs perfect. My guess (and I´m not an expert in computer architecture, so I might be very wrong) is that since the address $000F can´t be read, the data bus isn´t updated, so the subtract is made with $0F which is the last value loaded into the bus. Can anyone confirm me this? What is the purpose of this weird subtraction? Does anybody knows if there is another game with this behavior? 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? Thanks in advance, Nicolás - Archives (includes files) at http://www.biglist.com/lists/stella/archives/ Unsub & more at http://www.biglist.com/lists/stella/
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