Extending Ben Eater’s CPU with Stack Mechanism and Function Call Support

Why add a stack and function call support?

In C-like languages, function calls require:

• Saving the return address
• Storing local variables and parameters
• Executing CALL function and RET (return)

To support this, the CPU must have:

• A Stack Pointer (SP) register
• A reserved memory area used as the stack
• New instructions: CALL, RET, PUSH, POP

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New Instructions (Stack & Call Support)

Opcode	Mnemonic	Description
0x60	CALL addr	Push return address to stack, then jump to addr
0x61	RET	Pop return address from stack and jump back
0x62	PUSH A	Push the contents of register A onto the stack
0x63	POP A	Pop the top of the stack into register A
0x64	PUSH B	Push the contents of register B onto the stack
0x65	POP B	Pop the top of the stack into register B

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Hardware Requirements

• Add a new SP (Stack Pointer) register, e.g., using a 74LS173 or 74LS161
• Stack grows downward, from 0xFF to 0xF0
• Reserve upper memory for stack (e.g., 0xF0–0xFF)

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Example: Simple Function Call

C-like logic:

cCopyEditvoid sub() {
    mem[0x20] = 42;
}

int main() {
    sub();
    return 0;
}

Corresponding DCPU-8-like code:

assemblyCopyEdit00: 60 08     ; CALL 0x08 (subroutine)
02: FF        ; HALT

; subroutine at 0x08
08: 10 2A     ; Load A with immediate 0x2A (e.g., via workaround or memory preset)
0A: 20 20     ; STORE A, [0x20]
0C: 61        ; RET

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Benefits of Adding Stack + CALL/RET

• Enables modular programming and code reuse
• Simulates local variables and nested calls
• Lays foundation for recursive functions and future C support