add reference files for new skill legacy-circuit-mockups

2026-02-23 20:05:12 +00:00 · 2026-01-19 20:26:57 -05:00
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+# 6502 Assembly Language with AT28C256 EEPROM
+
+A practical specification for writing **6502/65C02 assembly language programs** intended to be stored in and executed from an **AT28C256 (32 KB) parallel EEPROM** in single-board computers (SBCs) and retro systems.
+
+---
+
+## 1. Scope and Assumptions
+
+This document assumes:
+
+* A **6502-family CPU** (6502, 65C02, or compatible)
+* Program code stored in an **AT28C256 (32K x 8) EEPROM**
+* Memory-mapped I/O (e.g., 6522 VIA)
+* Reset and interrupt vectors located in EEPROM
+* External RAM mapped elsewhere (e.g., 62256 SRAM)
+
+---
+
+## 2. AT28C256 EEPROM Overview
+
+| Parameter      | Value               |
+| -------------- | ------------------- |
+| Capacity       | 32 KB (32768 bytes) |
+| Address Lines  | A0-A14              |
+| Data Lines     | D0-D7               |
+| Access Time    | ~150 ns             |
+| Supply Voltage | 5 V                 |
+| Package        | DIP-28 / PLCC       |
+
+### Typical Memory Map Usage
+
+| Address Range | Usage                   |
+| ------------- | ----------------------- |
+| `$8000-$FFFF` | EEPROM (code + vectors) |
+| `$FFFA-$FFFF` | Interrupt vectors       |
+
+---
+
+## 3. 6502 Memory Map Example
+
+```
+$0000-$00FF  Zero Page (RAM)
+$0100-$01FF  Stack
+$0200-$7FFF  RAM / I/O
+$8000-$FFFF  AT28C256 EEPROM
+```
+
+---
+
+## 4. Reset and Interrupt Vectors
+
+The 6502 reads vectors from the **top of memory**:
+
+| Vector  | Address       | Description            |
+| ------- | ------------- | ---------------------- |
+| NMI     | `$FFFA-$FFFB` | Non-maskable interrupt |
+| RESET   | `$FFFC-$FFFD` | Reset entry point      |
+| IRQ/BRK | `$FFFE-$FFFF` | Maskable interrupt     |
+
+### Vector Definition Example
+
+```asm
+        .org $FFFA
+        .word nmi_handler
+        .word reset
+        .word irq_handler
+```
+
+---
+
+## 5. Assembly Program Structure
+
+### Typical Layout
+
+```asm
+        .org $8000
+
+reset:
+        sei             ; Disable IRQs
+        cld             ; Clear decimal mode
+        ldx #$FF
+        txs             ; Initialize stack
+
+main:
+        jmp main
+```
+
+---
+
+## 6. Essential 6502 Instructions
+
+### Registers
+
+| Register | Purpose          |
+| -------- | ---------------- |
+| A        | Accumulator      |
+| X, Y     | Index registers  |
+| SP       | Stack pointer    |
+| PC       | Program counter  |
+| P        | Processor status |
+
+### Common Instructions
+
+| Instruction | Function               |
+| ----------- | ---------------------- |
+| LDA/STA     | Load/store accumulator |
+| LDX/LDY     | Load index registers   |
+| JMP/JSR     | Jump / subroutine      |
+| RTS         | Return from subroutine |
+| BEQ/BNE     | Conditional branch     |
+| SEI/CLI     | Disable/enable IRQ     |
+
+---
+
+## 7. Addressing Modes (Common)
+
+| Mode      | Example       | Notes        |
+| --------- | ------------- | ------------ |
+| Immediate | `LDA #$01`    | Constant     |
+| Zero Page | `LDA $00`     | Fast         |
+| Absolute  | `LDA $8000`   | Full address |
+| Indexed   | `LDA $2000,X` | Tables       |
+| Indirect  | `JMP ($FFFC)` | Vectors      |
+
+---
+
+## 8. Writing Code for EEPROM Execution
+
+### Key Considerations
+
+* Code is **read-only at runtime**
+* Self-modifying code not recommended
+* Place jump tables and constants in EEPROM
+* Use RAM for variables and stack
+
+### Zero Page Variable Example
+
+```asm
+counter = $00
+
+        lda #$00
+        sta counter
+```
+
+---
+
+## 9. Timing and Performance
+
+* EEPROM access time must meet CPU clock requirements
+* AT28C256 supports ~1 MHz comfortably
+* Faster clocks may require wait states or ROM shadowing
+
+---
+
+## 10. Example: Simple LED Toggle (Memory-Mapped I/O)
+
+```asm
+PORTB = $6000
+DDRB  = $6002
+
+        .org $8000
+reset:
+        sei
+        ldx #$FF
+        txs
+
+        lda #$FF
+        sta DDRB
+
+loop:
+        lda #$FF
+        sta PORTB
+        jsr delay
+        lda #$00
+        sta PORTB
+        jsr delay
+        jmp loop
+```
+
+---
+
+## 11. Assembling and Programming Workflow
+
+1. Write source (`.asm`)
+2. Assemble to binary
+3. Pad or relocate to `$8000`
+4. Program AT28C256 via T48 / minipro
+5. Insert EEPROM and reset CPU
+
+---
+
+## 12. Assembler Directives (Common)
+
+| Directive  | Purpose                     |
+| ---------- | --------------------------- |
+| `.org`     | Set program origin          |
+| `.byte`    | Define byte                 |
+| `.word`    | Define word (little-endian) |
+| `.include` | Include file                |
+| `.equ`     | Constant definition         |
+
+---
+
+## 13. Common Mistakes
+
+| Issue                      | Result             |
+| -------------------------- | ------------------ |
+| Missing vectors            | CPU hangs on reset |
+| Wrong `.org`               | Code not executed  |
+| Using RAM addresses in ROM | Crash              |
+| Stack not initialized      | Undefined behavior |
+
+---
+
+## 14. Reference Links
+
+* [https://www.masswerk.at/6502/6502_instruction_set.html](https://www.masswerk.at/6502/6502_instruction_set.html)
+* [https://www.nesdev.org/wiki/6502](https://www.nesdev.org/wiki/6502)
+* [https://www.westerndesigncenter.com/wdc/documentation](https://www.westerndesigncenter.com/wdc/documentation)
+* [https://en.wikipedia.org/wiki/MOS_Technology_6502](https://en.wikipedia.org/wiki/MOS_Technology_6502)
+
+---
+
+**Document Scope:** 6502 assembly stored in AT28C256 EEPROM
+**Audience:** Retrocomputing, SBC designers, embedded hobbyists
+**Status:** Stable reference