CPU8 Plus 4+

CPU8Plus is an updated extension of the basic CPU8 App. Most of the extensions deal with the amount of ‘memory’ available for programs within this little 8-bit computer.

CPU8 teaches the user how computers work internally. It does this with a simple instruction set of just 16 instruction along with an 8-bit bus; two 8-bit registers; an 8-bit ALU; 8-bit storage word but only provides 4-bit addresses (that is 4-bit Program Counter and 4-bit Address Register) limiting the addressable memory space to just 16 bytes (or words).

CPU8Plus adds 4-bits to the address space via a new 8-bit Program Counter and Address Register. This is a major upgrade that provides 256 bytes (or words) of address space for programs and data — 16 times that of CPU8.

To facilitate this change, the microcode now decides if an instruction is a 1 or 2 word instruction and advances the PC accordingly while utilizing the larger space for the operand that has doubled from 4 bits to 8 bits.

The CPU8Plus model provides these changes and the assembler now generates new executable object code to reflect these changes. This is all done while maintaining source code compatibility. That means users of CPU8 can cut their source code from CPU8 and paste it into new files on CPU8Plus, assemble and run without any other changes. A common goal in the computer world.

Programs can be significantly more complex, however, since they can now be 16 times bigger.

Documentation has also been updated along with some history background. Most progress displays are now reported in hexadecimal rather than binary which has become too unwieldy. A separate “memory” display displays contents of the full 256 bytes.

Hopefully, this will enhance the learning process for you and provide new information about how computers work!

Suitable for classroom study, independent study or just for the sheer enjoyment of playing with a computer that you can watch and learn from. No individual information is accessed.