Safe page zero addresses

Vic40 · Post by **Vic40** » Fri Feb 15, 2008 3:38 pm

By it's good to have the forum back - been a bit lost over the last week without being able to call upon help.

Just wondering if there are any further page zero addresses that can safely be used apart from the two ($fb and $fd) which are listed as free in the Vic20 Programmers Reference Guide. I will be calling machine code routines from a Basic program which I guess will restrict the options just a little.

carlsson · Post by **carlsson** » Mon Feb 18, 2008 1:47 am

Actually those are four bytes: $FB, $FC, $FD, $FE. In theory $FF is also available, although some routines from Basic may use it.

I don't know for sure, but you could try to use the bytes för RS232 data transfer: $F7, $F8, $F9, $FA.

I'd avoid messing with FAC, AFAC, temporary storage for those and other utility/index locations in case Basic uses them.

Mike · Post by **Mike** » Mon Feb 18, 2008 4:11 am

There some ZP addresses shared between Tape/RS232 operations.

These are: $A7 - $AB, $B4 - $B6, $BD.

They are re-initialised (and thus overwritten) when you start a tape operation (like LOAD, SAVE, or OPEN) or a RS232 transfer. BASIC doesn't rely upon finding old values there, so they're free as long as there are no active Tape/RS232 operations.

Greetings,

Michael

Vic40 · Post by **Vic40** » Mon Feb 18, 2008 5:31 pm

Thanks to both of you for the "pointers".

My program won't be using any tape/RS232 operations so I should be safe to use them.

carlsson · Post by **carlsson** » Tue Feb 19, 2008 2:50 am

In the sticky ROM calls thread, there was some discussion about ZP addresses too, but it was based upon pure machine language programs, which addresses would be possible to reuse and possibly exit to Basic afterwards. In your case it is a little more restricted.