Instruction Set

Here's my take on the instructions set that will control the 4-bit TTL CPU.

Since the instruction bus is only 4 bits wide there can only be a maximum of 16 instructions. Each 4-bit operator is paired with a 4-bit operand so each instruction will make up 8 bits in total. The operator will be interpretted by the control logic and the operand will be processed. The chart below describes the instruction set:

Instruction OpCode Summary Description
LOAD (Mem) 0000 Accum = mem[Operand]; Load data to the accumulator from a value held in RAM with the address set by the operand.   
LOAD (Value) 0001 Accum = Operand; Load the operand data to the accumulaor.
ADD (Mem) 0010 Accum += mem[Operand]; Add value held in memory to the accumuator
ADD (Value) 0011 Accum += Operand; Add operand to the accumuator 
SUB (Mem) 0100 Accum -= mem[Operand]; Subtract value held in memory from the accumuator 
SUB (Value) 0101 Accum -= Operand; Subtract operand from the accumuator  
STO (Mem) 0110 mem[mem[Operand]] = Accum;  Store accumulator's value to memory, address held in memory.  
STO (Value) 0111 mem[Operand] = Accum; Store accumulator's value to memory, operand equals address. 
READ (Mem) 1000 Addr = mem[Operand]; Read from address, address held in memory.
READ (Value) 1001 Addr = Operand;  Read from address, operand equals address.
JMPLOW (Mem) 1010 PC Low Addr = mem[Operand] Jump the lower 4-bits of the Program Counter to an address held in memory.
JMPLOW (Value) 1011 PC Low Addr = Operand Jump the lower 4-bits of the Program Counter to an address held by the operand.
JMPHIGH (Mem) 1100 PC High Addr = mem[Operand]  Jump the upper 4-bits of the Program Counter to an address held in memory. 
JMPHIGH (Value) 1101 PC High Addr = Operand  Jump the upper 4-bits of the Program Counter to an address held by the operand.
JMPZERO (Mem) 1110 if(Accum == 0) {
    PC = mem[Operand];
}
If accumulator equals zero, program counter address equals a value in program memory.
JMPZERO (Value) 1111 if(Accum == 0) {
    PC = Operand;
}
If accumulator equals zero, program counter address equals the operand. 

* Since I've made my Address Bus 8-bits wide, I could really use 18 instructions in total. As it is I will have to make due with JMPZERO being confined to a 4-bit address space.