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According to the datasheet, it's comparable in time to a normal memory access. But instead of accessing 16-bits of memory, you only get 1 bit of CRU. That's why it's slower. If you want to transfer 16bits, it takes 16 times as long as a memory mapped port which is why I think it is slow, now many I/O ports may not require high speed so slow I/O is acceptable, but it stops the processor from executing the main program which must slow things down.

I do not have the skills and knowledge to design this yet, which is why I started this thread, in the hope that others could do it, but I thought that the powertran cortex circuit could be adapted for this, I am very new to the 99/4a and do not yet understand the complexities of the operating system, I only have a console so I cannot be sure that anything I come up with will work with anything else that might be attached, this must be a community project to ensure it works with any hardware. The limiting factor in this idea is the availability of the 9995 processor which are getting difficult to find or expensive.

It does not need them, the accelerator acts like a cartridge passing instructions and data to the 9900, whilst the application program is actually executed by a 9995 in the cartridge, the 9900 in the console controls all the hardware attached to the 99/4a and moves data to/from the cartridge as dictated by the program running on the 9995. The application program will be loaded into the cartridge by the 9900 from CF, disk or cassette, so a small rom is needed in the cartridge to provide simple BIOS functions.

Geneve is not designed to plug into the cartridge port, but needs an expensive and in the UK, unobtainable PEB, the geneve cannot be bought now and the geneve 2 is not yet available. The Console on its own is not very useful, but an accelerator like this would give it a whole new lease on life.

CRU I/O is slow and stops the processor from accessing memory during CRU operations, anyone know how much this impacts on performance?

Many other computers have had accelerators and processor upgrades designed for them, would there be any interest for one in the TI 99/4a community?, I do not have the skills to design it myself (although that won't stop me from trying to do it one day). I thought about having an accelerator cartridge that plugged into the cartridge port, it would contain a 9995 based single board computer with 1MB static ram (2 chips), maybe a CF or SD socket, and it would use the 9900 in the console, as an in/out processor, moving data between the accelerator cartridge and the devices attached to the 99/4A such as the keyboard, screen and disk drive Etc. Programs running on the accelerator would not be affected by the crippled architecture of the 99/4A, it might even be possible to copy GPL and a modified BASIC into the cartridge, although that might slow things down too much, but a simple monitor program will allow programs to access the 99/4a hardware via the 9900, there would also need to be code that tells the 9900 how to be the I/O processor.

I read that interview and I thought he was saying the engineers wanted TI to design a processor that ran GPL as its machine code, this processor would have replaced the 9900 in the 99/4B, but management chose not to develop such a chip.