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November 14, 2018, 02:09:46 AM *
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Author Topic: Using the so called "External" RAM?  (Read 31 times)
Trying to get the cartridge in...
Posts: 25

« on: November 13, 2018, 01:36:19 PM »

So, a really stupid question:
I've run out of regular memory, (0x20-0x3f). I've read in some of the documentation that there is something called "External RAM", that can be accessed by somehow switching so that the MOVX command works on that instead of on the VDC.
* Is these 128 bytes of "external" RAM present in all consoles (so the "external" only means external to the CPU) or is it something that is on some carts only (external as "on the cartridge")?
* To access this memory, do you simply need to "orl P1,#008h", then use movx on addresses between 00-7f and then you do a "anl P1,#0f7h" to switch back to VDC-access?
Give this man a Jopac!
Posts: 1112

« Reply #1 on: November 13, 2018, 08:39:51 PM »

If you're not using the quads' (0x40-0x7f), you can use those as well.

All my Odyssey2 projects, now in one place!
Played Munchkin once...
Posts: 70

« Reply #2 on: November 13, 2018, 09:08:31 PM »

Yes, the external RAM is present on all consoles. You certainly can use it to store any data you like, but in the original carts it's primarily used for updating the screen during vblank in a table-driven method. This prevents flicker, especially if you have a lot of things moving around and graphics to update.
Attacked the Timelord
Posts: 122

« Reply #3 on: Today at 02:03:26 AM »

Don't use bytes $3D, $3C and $3F from internal RAM as BIOS uses them to store timing, collision and video status.
You can set the extra ram and vdc using BIOS calls, if your code is in the first 1k:

vdcenable   equ   00E7h
extramenable   equ   00ECh

You can check these codes (assuming you have BIOS disassembly) to know exactly how to set the pins.
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