$d301 default value

[Heaven/TQA]

what is the default value of $d301?

 

thanks...

[analmux]

...that depends on which system you're using.

 

$d300 and $d301 are general bidirectional multipurpose ports

 

$d301 is joystick 3&4 on the atari 400/800 line, so has no default value (as it is read only)

 

$d301 on the 600xl/800xl/65xe etc. line is write only and has value $fd =253

 

bit 0 on means OS-ROM enabled

bit 1 off means Basic-ROM enabled

 

so $fe (254) will turn ALL ROM off. And when you do so, be sure to disable all kinds of interrupts (IRQ/NMI) and then replace the vectors at $fffa,$fffe

[analmux]

Oh yes...bit 7 on means selftest ROM at $5000-$57ff disabled.

 

funny, I tried once if the 2kb invisble RAM from $d000-$d7ff would apear on the selftest area ($5000-$57ff) when selftest ROM (part of OS-ROM mapped 32 kb back in memory) enabled but OS-ROM turned off:

 

bit 7=0, bit 0=0: $7e or 126

 

No effects....

[analmux]

....and with control registers $d302, $d303 (bit 2 turned off/on?) you can reach the data-direction registers: now $d300 and $d301 are used for setting the R/W direction on the same registers.