$0000/$0001 data direction registers - VIC20?

[MIRKOSOFT]

Hi!

I'm C128 and C64 programmer and both machines have $0000 and $0001 as data direction registers.
They're different in some parts but function is same.
My Q is:
C64 allows to control Kernal and Basic ROM overlay in RAM by $0001.

What a function have these addresses in VIC20 and is possible anything like in C64 with Kernal and Basic overlay?

Thank you for your support.
I'm on VIC20 beginner.

Miro

[srowe]

Zero page locations $0000 and $0001 are plain memory on the VIC-20. They hold a $4C (JMP) and the LSB of a user-defined routine to be called by the USR() command. This is comparable with the addresses $0310 and $0311 on the C64.

[MIRKOSOFT]

Ok, understand. So, no any direction register.

Miro

[Mike]

Just to set straight the terminology used here:

$00 and $01 on the 6510/7501/8502 provide the CPU I/O port. Of these two registers, $00 is the data direction register, $01 is the port data register. The 6502 CPU, used in the VIC-20, doesn't feature this CPU I/O port.

Neither, for that matter, does an unmodified VIC-20 provide the capability to have RAM 'under' BASIC, KERNAL and character ROM. This has just recently been discussed here in Denial, though for other reasons.

Further info:

• "Ultimem BLK6",
• "Custom Char Set and Memory", and
• "Upgrade a VIC 20 to a C64, possible via expansion slot?".

Greetings,

Michael