Internal CPU registers:

Program Counter $($PC$)=$ Address of instruction

Instruction Register $($IR$)=$ Instruction being executed

Accumulator $($AC$)=$ Temporary Storage

Partial list of opcodes:

$0001=$ Load AC from memory

$0010=$ Store AC to memory

$0101=$ Add to AC from memory

$0011=$ Load AC from I$/$ODevice

$0111=$ Store AC to I$/$ODevice

$1000=$ Add to memory from AC