Intel 4004 microprocessor architecture

Memory

Program memory size is 4 KB. All conditional instructions work within currently selected ROM (256 bytes). Unconditional jump and jump to subroutine instructions can be used to jump to any address.

Data memory size is 640 bytes. RAM access is done in the same way as access to I/O ports. First, a SRC instruction is used to tell the processor what memory address to access, and successive WRM or RDM writes accumulator data to memory or reads data into accumulator. Data memory is separate from program memory.

Stack is 3-level deep (3 x 12-bit registers). Stack is separate from program memory and data memory.

Interrupts

None.

I/O ports

16 4-bit input ports.
16 4-bit output ports.

Registers

Program counter (12-bit).

Stack registers. There are three 12-bit stack level registers, which is sufficient to implement 3-level deep subroutine calls. Every subroutine call stores return address in one of the stack registers. Contents of the accumulator and the index registers is not preserved.

Accumulator (4-bit). Mainly used for arithmetic and logic operations, as well as for reading and writing data from/to RAM and I/O ports.

Index registers. The microprocessor has 16 4-bit registers, that can work in pairs as 8 8-bit registers.

Instruction Set

4004 instruction set consists of 46 instructions:

• Data moving instructions.
• Arithmetic - add, subtract, increment, decrement.
• Logic - rotate.
• Control transfer - conditional (limited to current ROM), unconditional, call subroutine and return from subroutine.
• Input/Output instructions.
• Other - carry flag operations, decimal adjust, etc.

Instruction length can be one or two bytes.

Addressing modes

Register (4-bit).

Memory direct.

Register indirect. Register indirect can access data only within current ROM.

Immediate (4 and 8-bit data).