DCPU-8 Stock-able Computer

✅ Project Goal

Create a modular stackable 8-bit computer, compatible with:

• DCPU-8 v0.1 (your custom ISA)
• Stack-based function calls (CALL, RET, PUSH, POP)
• Plug-and-play PCBs that connect via a 40-pin backplane

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🧱 Module Overview

Each module is a separate PCB. They all plug into a main bus board or stack vertically with headers.

PCB Module	Function	Features
1. Clock	555 Timer / Manual button / Speed switch	Step/Run modes
2. Control Unit	Microcode EEPROM sequencer	ROM-based finite state controller
3. Registers	A, B, IR, MAR, OUT, SP	74LS173 or 74LS161
4. ALU + Flags	Basic ALU (ADD, SUB, AND, OR, XOR) + Z	Using 74LS181 or discrete gates
5. Memory	EEPROM (program) + SRAM (data)	28C256 + 62256 or similar
6. Stack Module	Stack Pointer, Stack RAM access	SP register, decrement/increment logic
7. Output Display	LEDs or 7-segment from OUT register	Visual debug
8. System Bus	Backplane or vertical headers	Shared control/data/address lines

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🔌 Shared Bus Design

A 40-pin dual-row connector (2×20) can be used between all boards. Here’s a typical pin assignment:

Pin Range	Signal Type	Notes
0–7	Data Bus (D0–D7)	Bidirectional
8–15	Address Bus (A0–A7)	Unidirectional
16	CLK	Clock
17	RST	Reset
18	Z Flag	Zero Flag
19	C Flag	Carry Flag
20	IR Enable	Instruction Register load
21–29	Custom Control Lines	e.g., Memory Enable, Write, etc.
30–31	GND, Vcc	Power lines

You can route these on a vertical header or horizontal backplane (à la RC2014).

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🔁 Stack & CALL/RET Implementation

• Stack Pointer (SP) register uses a counter IC (e.g., 74LS193) and is mapped to a dedicated RAM range (e.g., 0xF0–0xFF)
• CALL pushes return address onto stack, then jumps
• RET pops return address back into PC
• PUSH and POP work with A/B registers and SP

These are microcoded in the Control Unit using EEPROM (e.g., 28C16/28C64).

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💾 Program Example: DCPU-8 Adder with CALL

Let’s say your ROM contains this:

assemblyCopyEdit00: 01 10     ; LOAD B, [0x10]
02: 00 11     ; LOAD A, [0x11]
04: 40        ; ADD A, B
05: 20 12     ; STORE A, [0x12]
07: 60 0A     ; CALL function @ 0x0A
09: FF        ; HALT

; Subroutine
0A: 10 2A     ; A = 0x2A (load literal or via [mem])
0C: 20 13     ; STORE A, [0x13]
0E: 61        ; RET

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🛠️ Tools You’ll Use

Tool	Use
KiCad / EasyEDA	Schematic and PCB layout
JLCPCB / OSH Park	Manufacturing boards
28C256 / 62256	EEPROM and RAM (program & data)
74LS / 74HC series	Logic chips (registers, ALU, control)
Header pins & stack spacers	Connect modules in stack/backplane

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🚀 Suggested Build Order

1. Clock Board – verify manual & automatic clock control
2. Bus Board – shared headers, power rails
3. Registers Board – implement A, B, OUT
4. Memory Board – EEPROM for program + SRAM for data
5. Control Unit – EEPROM-based microcode sequencer
6. ALU Board – logic for ADD, SUB, AND, etc.
7. Stack Module – with SP + stack memory
8. Output Board – LED or display from OUT register