Usage

Enter a program by clicking on the dots in the 8x8 memory grid.
Use the buttons at the bottom to step or run the program.
Reset will cause the PC and the register to go to zero.

How it works

The C88 has a RAM which has eight locations, each location contains an eight bit word.
This RAM is for both program and data.
There is also an eight bit register, known simply as "the register".
To explain; here is an example program that adds two numbers together.

0 - Load 4
1 - Add 5
2 - Store 6
3 - Stop
4 - 23
5 - 15
6 - (output location)
7 - (unused)

The first instruction loads the number 23 in to the register from memory location 4.
The second instruction adds 15 to the register and stores the result in the register. (15 is the value of memory location 5.)
Then the result is stored in memory location 6.
The program then stops.

Try it here.

It's that simple.
The IO stuff is only really useful if you have a physical implementation of the machine.

Some more interesting programs

Die roll. Run fast and then stop to generate a random number in address 7.
GPIO Die roll. Same as above but outputs to GPIO register and is not biased towards six.
Cylon. Run slow to see a cylon flash cycle in the main register.
Slider animation. Like the cylon but the edge between lit and unlit moves instead of a single lit bit moving.
Integrator. Takes the GPIO input, integrates it, puts the result on the GPIO output.
Differentiator. Like the Integrator, but does a differentiation instead. More useful with a physical machine that has some IO stuff attached.

Instruction encoding

Each C88 instruction is eight bits wide. The five most significant bits (the leftmost five bits) are the opcode. The three least significant bits are the address or operand.

Most instructions take an address. The shift and rotate functions take one operand (between 0 and 7).
The Inc, Dec, Double, Half, Stop and IO instructions require no address or operand.

• 00000 LOAD Load an address into the register
• 00001 SWAP Swap the register value and the value at some address
• 00010 STORE Store the register into an address
• 00011 STOP Stop the program
• 00100 TSG Test, skip if greater
• 00101 TSL Test, skip if less
• 00110 TSE Test, skip if equal
• 00111 TSI Test, skip if inequal
• 01000 JMP Jump to specified address
• 01001 JMA Jump to the address stored at the specified address
• 01010 (Not encoded) Use of this op code causes undefined behaviour. This emulator will halt on this instruction.
• 01011 (Not encoded)
• 01100 IOW IO write, write register to output
• 01101 IOR IO read, read input to register
• 01110 IOS IO swap, write register to output then load input to register
• 01111 IOC IO clear, write 0 to output register
• 10000 ADD Add value at address to register, result in register
• 10001 SUB Subtract value at address from register, result in register
• 10010 MUL Multiply value at address by register, result in register
• 10011 DIV Divide register by value at address, result in register
• 10100 SHL Shift register left by amount specified (as address)
• 10101 SHR Shift register right by amount specified (as address)
• 10110 ROL Rotate register left by amount specified (as address)
• 10111 ROR Rotate register right by amount specified (as address)
• 11000 ADDU Same as ADD but unsigned (same as ADD, double encoded)
• 11001 SUBU Same as SUB but unsigned (same as SUB, double encoded)
• 11010 MULU Same as MUL but unsigned
• 11011 DIVU Same as DIV but unsigned
• 11100 INC Increment register by one
• 11101 DEC Decrement register by one
• 11110 DOUBLE Double the value of the register
• 11111 HALF Half the value of register

Copyright and licence

© Lex Bailey 2015 (Machine spec, Simulator GUI and instructions, Die roll program, GPIO Die roll, integrator, differentiator, slider)
© Lissa Moriarty 2015 (Simulator back end, Cylon program)

You are permitted to redistribute this page and associated code, with or without modification, provided you give proper attribution to the original authors.