3 more instructions

A project log for TMS9900 compatible CPU core in VHDL

Retro challenge 2017/04 project to create a TMS9900 compatible CPU core. Again in a month... Failure could be an option...

Erik Piehl 04/25/2017 at 20:390 Comments

Today was a busy day in the office - I just had the time and energy to add 3 more instructions. They are from a new category - one that only allows a workspace register as a destination parameter, while the source operand can have all addressing modes, for example:

This category has five instructions in total (DIV and MPY are still missing) but COC, CZC and XOR are now done. Of these instructions XOR is the most familiar and supported by virtually all processors - it just does the XOR operation of source and destination and stores the result to destination while also setting 3 status flags.

COC and CZC are unusual instructions, I have not seen these on any other processor although I have programmed in assembler on many CPUs.

COC stands for "compare ones corresponding" and CZC stands for "compare zeros corresponding". Since they are comparison instructions, there is no actual data output other than the result of comparison which is stored in the zero flag.

I implemented both using new ALU operations, in VHDL as below. I don't think I have ever used these instructions, so this implementation follows from what I understood from the TMS9900 data sheet.

COC: alu_out <= ('0' & arg1 xor ('0' & arg2)) and ('0' & arg1);

CZC: alu_out <= ('0' & arg1 xor not ('0' & arg2)) and ('0' & arg1);

(The extra zero bits '0' are just garbage in the above to account for the fact that the ALU is actually 17 bits wide, in order to be able to generate the carry flag - which is not used by these instructions).

Both of these comparison instructions take the source operand (arg1 above) and make sure that the result indicates that in the destination operand there are one bits (for COC, or zero bits for CZC) in each location where there are one bits for the source. I did the core of the comparison with XOR, and arg1 is used as a bit mask to leave only the relevant bits. The standard result comparison to zero in the flag creation logic works.

I'm not really sure why they thought around 1977 that these are useful instructions... I can only assume they originated from the minicomputer architecture. These operations can so simply be implemented with basic boolean operations. Clearly this architecture was not designed with C compilers in mind - but that is evident from many other things as well, such as the lack of a proper hardware supported stack.

Whatever - 3 more instructions done and a little tested - and only 4 instructions remain!