"Patrulha Espacial" Patch for 50Hz ZX81

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Fruitcake
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"Patrulha Espacial" Patch for 50Hz ZX81

Post by Fruitcake » Sun May 27, 2018 12:04 pm

Kelly Murta has made available some excellent games here: http://zx81.eu5.org/

All games were originally released for the Brazilian TK85 clone of the ZX81.

One of the games is 'Patrulha Espacial' (Space Patrol), which is a cracking shoot 'em up style game. This can be played with the EightyOne emulator but only if configured as a TK85 or as a 60Hz ZX81 ('NTSC TV' option in EightyOne). This is because the game reads the keyboard port but does not mask off the top 3 bits. Bit 6 indicates whether the ZX81 is set to 50Hz or 60Hz operation, and the game expects it to be 0, i.e. 60 Hz operation.

I've produced a patched version that allows it to work on any ZX81. There is an unused area of memory in the program (which looks to be the unused end of a REM statement that holds the machine code) and so I have placed a patch here that discards the upper 3 bits and then sets bit 5 to 1. This results in values that the game is expecting. The changes are as follows:

$450D: CALL $5EC5

$5EC5: AND $1F
$5EC7: OR $20
$5EC9: LD ($4503),A
$5ECC: RET

And here is the patched version of the game:
Patrulha Espacial (patched).p
(10.95 KiB) Downloaded 75 times

dr beep
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Re: "Patrulha Espacial" Patch for 50Hz ZX81

Post by dr beep » Sun May 27, 2018 4:09 pm

Nice.

McKlaud
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Re: "Patrulha Espacial" Patch for 50Hz ZX81

Post by McKlaud » Sun May 27, 2018 9:42 pm

Interesting stuff. :mrgreen:
Claudius
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RobertK
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Re: "Patrulha Espacial" Patch for 50Hz ZX81

Post by RobertK » Fri Jun 01, 2018 11:00 am

When I tried "Patrulha Espacial" and "Missão Andrômeda" I was surprised to hear my Chroma Interface-equipped ZX81 play music over the TV! I don't have any sound equipment, so that was like when your cat suddenly starts talking to you...

Activate "Beeper Sound" in the EightyOne emulator, then you will hear the intro tune.

I assume these two games were designed for the TK-82 which seems to have a beeper where you can hear the tape I/O sounds, and that's how the music is probably generated (the TK-85 even has a three-voice sound generator).

Can the sound be heard on a standard ZX81 by connecting headphones or active speakers? If so, to which port, MIC or EAR?

Fruitcake
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Re: "Patrulha Espacial" Patch for 50Hz ZX81

Post by Fruitcake » Fri Jun 01, 2018 8:44 pm

RobertK wrote:
Fri Jun 01, 2018 11:00 am
I don't have any sound equipment, so that was like when your cat suddenly starts talking to you...
:lol:

RobertK wrote:
Fri Jun 01, 2018 11:00 am
Activate "Beeper Sound" in the EightyOne emulator, then you will hear the intro tune.
Doing this will also cause EightyOne to output a buzz when displaying a normal picture, which is caused by the TV sync pulses the ZX81 generates. Chroma does not output the TV sync pulses as sound if it determines that a standard TV picture is being generated. But when the ZX81 is performing another operation, e.g. loading/saving, or as in this case deliberately timing its IO output then it renders these as sound. EightyOne should probably be enhanced to exhibit this behaviour when it has Chroma support enabled....

RobertK wrote:
Fri Jun 01, 2018 11:00 am
I assume these two games were designed for the TK-82 which seems to have a beeper where you can hear the tape I/O sounds, and that's how the music is probably generated (the TK-85 even has a three-voice sound generator).
I'm not very familiar with the differences of the TK-8x range, so this is interesting to hear.

RobertK wrote:
Fri Jun 01, 2018 11:00 am
Can the sound be heard on a standard ZX81 by connecting headphones or active speakers? If so, to which port, MIC or EAR?
Yes, through the MIC socket. If you have a cassette recorder that has a monitor function then you can set it to record and you should hear the sounds.

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