So what will change ?
- Microcode is not used any more, instructions will be RISC
- Four new 16-bit data registers in hardware (now total 8 registers)
- The 8-bit ALU will change to 16-bit ALU
- All instructions need two cycles (fetch, execute)
The Kobold K2 will be almost twice as fast, and its operation will be easier to explain.
The video system will stay mostly the same.
Finding the balance between low number of parts and high functionality is one of the key aspects of TTL CPU design (at least, for me it is). I want to keep the part count low, but not to the extreme as in #1 Square Inch TTL CPU. The CPU part of the computer should fit on a single PCB.
To keep the control system simple, every instruction should execute in a single cycle. If the ALU was kept 8 bits wide, that would mean 2 instructions for many 16-bit actions (as in the Z80 or 6502), and that would slow down 16-bit operations. Therefore, the ALU is now 16 bit wide. I don't want to use the 74181 ALU, so to keep part count reasonable, the ALU has only a few functions. The small number of functions also simplifies control.
The average performance per clockcycle is expected to be higher than that of a 6502 or Z80 and might come close to the performance of a 68000 in several situations. The performance is mainly due to the RISC strategy, fast access to 4 data registers and 4 address registers, and to having everything 16 bit wide.
The pcb of the CPU is now (5 oct 2019) routed. It also gives an impression of the various CPU parts (ALU, Registers, Control), see also this log about the PCB.