I was hoping to complete the core in terms of instruction set today - but no such luck. But I did add a bunch of instructions:

• XOP - extended operation. More about this below.
• STST - store status word to a workspace register. I am kicking myself for not implementing this instruction before, as it enables very easy flag functionality verification: do a computation impacting flags, store flags into a register and do an immediate comparison. If mismatch, stop. The source material for the comparison needs to come from a genuine TMS9900 or more likely from the classic99 emulator - or from my TMS99105 based TI-99/4A clone. This should be an easy way to verify the behaviour of flags which is pretty involved on the TMS9900.
• STWP - store workspace pointer to a workspace register.
• IDLE, RSET, CKOF, CKON, LREX - These are so-called external instructions. They basically just show a status code on the bus. IDLE should also stop and wait for an interrupt but I am not doing that yet.

The XOP instruction turned out to be the real deal, a proper mega instruction. I was thinking earlier after adding the BLWP instruction, that it does not get more complex than that. The XOP is a kind of software interrupt, which transfers control via a table in address >0040. What is unique about the TMS9900 is that the XOP instruction support a parameter, and the effective address of the parameter is put into register R11. This really converts the instruction into a door to many useful and compact constructs. It is sad that in the design of the TI-99/4A ROM they did not really provide any provision for general use of this instruction.

An example: XOP *R3,3

This instruction activates the XOP number 3 (out of 16). The vector is calculated as >40+4*3, i.e. from >4C. From [4C] is loaded the new workspace pointer and from [4E] new program counter. Then no less than four values are stored into the new workspace: old values of PC, ST and W register, and finally the effective address of *R3 (which happens to be the contents of R3 which in this case would be a memory pointer).

I modified the processing of BLWP instruction to serve Reset, XOP and BLWP use cases - I suspect that once I implement interrupts they will also use the same internal states, since interrupts are effective a bit like XOPs or BLWPs, in that they also vector through a memory location, change context and save previous state into the new context.

After these instructions there are only seven instructions left to do! They are COC, CZC, XOR, MPY, DIV, LDCR and STCR. Of these I want the multiply instruction MPY to use the Xilinx FPGA DSP block for good performance. In addition interrupt support needs to go in, but that should be easy at this point due to the BLWP/XOP support.