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
.org $FFFA
.word nmi_handler
.word reset
.word irq_handler
5. Assembly Program Structure
Typical Layout
.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
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)
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
- Write source (
.asm)
- Assemble to binary
- Pad or relocate to
$8000
- Program AT28C256 via T48 / minipro
- 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
Document Scope: 6502 assembly stored in AT28C256 EEPROM
Audience: Retrocomputing, SBC designers, embedded hobbyists
Status: Stable reference