8 Bit Computing

The CPU is an 8-bit width computer architecture, meaning that the data buses, registers and arithmetic logic units can only process 8-bit data. Because each bit represents either a 0 or a 1, an 8-bit register can store 2⁸ unique values. In my design, integers are represented by unsigned binary numbers meaning that the 2⁸ values span from 0 to 255.

Data Bus

With the exception of the control section, the 8-bit data bus connects components of the CPU together to allow the transfer of information and data. Data is sent and received through the bus by enabling specific components and setting others. For instance, to store the output of an ADD operation from the ALU to register 0, the CPU would enable the accumulator onto the bus and set register 0.

Arithmetic Logic Unit

The Arithmetic Logic Unit (ALU) is a component of the CPU that processes arithmetic, bit shifting and other bitwise operations such as OR, AND, and NOT. The ALU is a combinational circuit, meaning that its output is a function of its current inputs and has no dependence on previous inputs. In other words, all potential operations are performed in parallel in the ALU and the desired one is selected by the opcode.

Opcodes & Operations

Opcodes, meaning operation codes, are used to instruct the ALU to perform a specific arithmetic or bitwise operation. In my implementation of the ALU, the opcode size is 3 bits, and a 3x8 binary decoder is used to map the opcode input into its respective operation.

Outputs

The main output of the ALU is the result of the arithmetic or bitwise operation. This value is stored in the accumulator that holds the result , which is a type of register.

The ALU also outputs status signals that represent carry out, zero, equal, and A larger. The status signals are stored in an external flag register where the codes are stored for use by the control section.

Registers

Registers are storage components of a CPU that are used to temporarily hold information such as instruction addresses and program output, sort of like variables in a function. There are many types of registers that serve specific purposes. My CPU design includes general purpose registers, an instruction register, an instruction address register, a memory address register, an accumulator, and a status register.

The internal workings of a general purpose register include gated SR (set-reset) latch memory followed by an enable AND gate and a controlled buffer. The general purpose registers have input, output, enable, and set pins. However, not all registers have enable pins or controlled buffers and enable gates. For example, the instruction register does not have an enable pin nor an output controlled buffer and its data is always available in the control section.

RAM & MAR

The Random Access Memory (RAM) was the most satisfying to build. Scalable to 256 bytes, I only implemented 32 bytes for simplicity and efficiency. Each block of memory is a 1 byte (8-bit) register.

The Memory Address Register (MAR) is connected to the bus and stores the address of either an instruction or data in temporary memory.

0000 0000

The stored address is read into two 4x16 decoders with the first four bits indicating the vertical position and the last four bits indicating the horizontal position of the desired memory location.

Clock

The clock generator is responsible for syncing all of the processes that occur in the CPU, such as the set and enable signals for many components. The CPU clock is powered by a J-K (Jack Kilby) flip flop and has three outputs—the original clock, the set clock, and the enable clock. The leading and falling edge of the set clock signal must fall between the leading and falling edges of the enable clock. This is exactly what the J-K flip flop circuit does. It takes 4 clock pulses to complete one instruction cycle.

Control Section

The control section is where all of the control logic resides. While not directly connected to the data bus, the control section is connected to every component, enabling and setting them according to the current instruction. To interpret 8-bits of instruction into physical processes is the remarkable job of this component. The outputs of the control section include the enable and set signals for all of the CPU’s components The inputs of the control section include the clocks, the load pin, the ALU flag (status) register output, a reset pin, the MAR set pin, and most importantly, the instruction itself.

The control logic within the component was the most challenging part of the project. Here are the instruction codes for this CPU design.

ALU Instruction

1OOORARB

OOO are the opcodes used to select the arithmetic or bitwise operation, RA represents the two binary bits of register A and RB represents the two binary digits of register B. The ALU instruction would load RB into the temporary register which feeds into ALU input B and then RA into ALU input A.

LOAD Instruction

0000RARB

RA represents the two binary bits of register A and RB represents the two binary bits of register B. The LOAD instruction loads register B from the RAM address in register A.

For example, to load data into register 3 from the RAM address held in register 2, the LOAD instruction would be 00001011.

STORE Instruction

0001RARB

RA represents the two binary bits of register A and RB represents the two binary bits of register B. The STORE instruction stores the contents of register B into the RAM address in register A.

For example, to load the data in register 2 into RAM at the address in register 1, the STORE instruction would be 00010110.

DATA Instruction

0010XXRB

XX are not used in this instruction. Only RB, the last two bits, are used to represent register B. The DATA instruction loads the data in the next RAM address into register B.

JUMP REGISTER Instruction

0011XXRB

XX are not used in this instruction. Only RB, the last two bits, are used to represent register B. The JUMP REGISTER instruction jumps in the RAM to the address in register B.

JUMP ADDRESS Instruction

0100XXXX

XXXX are not used in this instruction. The JUMP ADDRESS instruction jumps to the next address in the RAM. It does not load any data into any registers.

JUMP IF Instruction

0101CAEZ

CAEZ represent the flags from the ALU, carry out, A larger, equal, and zero. The following table explains the instructions for each combination of these flags.

CLEAR FLAGS Instruction

0110XXXX

XXXX are not used in this instruction. The CLEAR FLAGS instruction will clear the ALU flags CAEZ.