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Your understanding is basically correct, but the design purpose of macro code was not just speed. It was basically a follow-on to the complex instruction set of the TI990/12 mini computer and its user writable micro code. It was also a way to customize interaction with a MMU and a way station when developing co-processors. For a quick description of the 990/12 see the attachement. For a description of the full 990/12 instruction set, see here http://bitsavers.informatik.uni-stuttgart.de/pdf/ti/990/assembler/2270509-9701A_AsmRef_Nov82.pdf It would seem that the micro-code user guide is lost, unfortunately. With external macro rom and ram, it is possible to implement the full 990/12 instruction set on a 99000 CPU and to provide an alternative to user writable micro-code, using normal assembler code (plus a few extra instructions, like 'EVAD'). According to the contemporary materials there were 3 versions of the 99xxx: the 99000 with just LDD, LDS and LMF in internal macro rom (for the 990/10A mini), the 99105 without internal macro rom and the 99110 with the 990/12 single precision floating point instructions and LDD/LDS in the internal macro rom. There is a lot of mention of a planned 99120 with parts of the "Rx" micro-kernel of TI's "Microprocessor Pascal" in its internal rom, but this was most likely never produced. In reality it would seem that only 99000 and 99110 silicon was produced and that chips marked "99105" have either 99000 or 99110 silicon inside. We still need to test more 99105 chips to understand this better. I've modified Stuart's 99110 experimentation PCB (see http://www.stuartconner.me.uk/tms99110_breadboard.htm) to handle external macro ram and to see if we can access the internal macro rom to see what is there. The 99110 offers an extension feature which seems to make this possible. My current 99105 seems to have 99000 silicon and does not appear to have this extension feature. Another 99105 is on its way from China and I hope this will have 99110 silicon (chips marked 99110 seem to be very, very rare). According to contemporary documents, the last 16 words of internal macro rom contain code for factory testing the macro rom. My hope is that this code can be accessed on any 99xxx, which may provide a backdoor to get access to the macro rom as well (so that chips without the extension interface can be read out). Paul ti990_12.pdf
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