does the 32k sidecar ram expansion for the ti99 add to the 16k ram already on board the computer?

xxx · January 23, 2021

or does it just override it? so you only get 32k as opposed to 48k?

Edited January 23, 2021 by xxx

+arcadeshopper · January 23, 2021

or does it just override it? so you only get 32k as opposed to 48k?

Did you read the FAQ? It adds 32k to the cpu the 16k in the console is VDP RAM

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Airshack · January 23, 2021

The TI-99/4A is unlike any other popular computer of its era. You must understand that it was initially designed around an 8-bit CPU which failed in development. To save the project TI decided to slip a 16-bit CPU into a computer with an 8-bit data bus and 8-bit support hardware.

The console has 16k of VDP RAM. What’s the deal with calling it VDP RAM? This VDP RAM is not directly CPU addressable as the case with other vintage computers. The console’s stock 16k RAM requires the TMS9918A VDP to act as an intermediary between the CPU and that VDP RAM. The CPU must access VDP RAM through ports, or decoded address lines. TI went with this slower-RAM design approach because VDP RAM was much cheaper than the faster conventional RAM back in 1978. Notice how most early systems from that era shipped with only 4K or 8k of RAM. This computer predated the Commodore 64 and other 48/64k systems.

You’re asking about expansion RAM. Expansion RAM has nothing to do with the console’s VDP RAM — no overlapping. So 32k expansion RAM simply adds 32K of CPU addressable (conventional) RAM to the system. It is added yet very much separate from VDP RAM.

The TI-99/4A stock console is often criticized for only having 256 bytes of 16-bit CPU addressable RAM. That’s actually 256 more bytes of 16-bit CPU RAM than any other 8-bit computer in existence. The TI-99/4A also has sixteen 16-bit (relocatable) registers which is something that really blows away any 8-bit architecture.

So you’re probably wondering more about how the 32K RAM expansion works. Here is a memory map of the TI-99/4A which shows it with 64K of addressable memory total:

8K is used by the console ROM

8K is referred to as “lower” expansion RAM

8K is used for peripheral expansion addresses: disks, rs232, printers, etc

8K is used for command module (cartridges) GROM/RAM

- 4K of which is fixed

- 4K of which is page swappable with up to nine 4K pages

8K is used for memory mapped devices, plus 256byte of 16-bit CPU RAM

- Example: The VDP is memory mapped here using 8-bytes for

read/write ports.

24K is used for “upper” expansion RAM

64K = Total

So the 32K expansion RAM is the sum of what is known as “lower” plus “upper” expansion RAM. You’re getting an additional 32K of CPU addressable 8-bit RAM. This is not to be confused with VDP RAM. It’s nothing like adding 32K to more traditional pure 8-bit machines with 8-bit CPUs.