Re: 6510 banking question

From: Christer Palm (
Date: 2001-05-05 02:05:04

Professor Dredd wrote:
> Okay, it sounds like you're confirming what I
> suspected.
> PLA controls which devices the 6510 can see in the map
> (RAML/RAMH/ROML/ROMH/IO/CHARGEN). The user/program
> CANNOT directly manipulate the PLA. A hardware RESET
> signal applied to the PLA (technically -RESET, IIRC)
> forces the banking logic of the PLA into the
> ROML/ROMH/IO configuration.

The C64 address decode PLA is stupid - its outputs are only dependent on
the current inputs.
It does not even have a RESET input.

It is the fact that the 6510 on-chip port is turned into input mode at
RESET that results in the default bank configuration. When the 6510 port
is in input mode (=high impendance), the -LORAM, -HIRAM, and -CHAREN
pullup resistors will make these signals high. This makes the PLA select
the following map:

0000-9FFF: RAM
D000-DFFF: I/O

Christer Palm
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