AS6C62256 (32K x 8 SRAM) Emulation Specification
A technical Markdown specification for emulating the AS6C62256 / 62256-family static RAM, suitable for 6502-family emulators, SBC simulators, and memory subsystem modeling.
1. Scope
This document specifies functional behavior for emulating:
- Alliance Memory AS6C62256
- Compatible 62256 (32K x 8) SRAM devices
Out of scope:
- Analog electrical timing margins
- Bus contention and signal rise/fall times
- Power consumption characteristics
2. Chip Overview
Core Characteristics
| Feature | Value |
|---|---|
| Memory Type | Static RAM (SRAM) |
| Capacity | 32,768 bytes (32 KB) |
| Data Width | 8-bit |
| Address Width | 15-bit (A0-A14) |
| Access Type | Asynchronous |
| Supply Voltage | 5 V (typical) |
3. External Signals (Logical Model)
| Signal | Direction | Purpose |
|---|---|---|
| A0-A14 | Input | Address bus |
| D0-D7 | I/O | Data bus |
| CE# | Input | Chip enable (active low) |
| OE# | Input | Output enable (active low) |
| WE# | Input | Write enable (active low) |
#indicates active-low signals.
4. Address Space Mapping
- Address range:
0x0000-0x7FFF - Address lines select one byte per address
Typical 6502 System Mapping Example
| CPU Address Range | Device |
|---|---|
$0000-$7FFF |
AS6C62256 SRAM |
$8000-$FFFF |
ROM / EEPROM |
5. Read and Write Behavior
Read Cycle (Logical)
Conditions:
CE# = 0OE# = 0WE# = 1
Behavior:
D[7:0] memory[A]
If OE# = 1 or CE# = 1, data bus is high-impedance.
Write Cycle (Logical)
Conditions:
CE# = 0WE# = 0
Behavior:
memory[A] D[7:0]
OE#is ignored during writes- Write occurs on active WE#
6. Control Signal Priority
| CE# | WE# | OE# | Result |
|---|---|---|---|
| 1 | X | X | Disabled (Hi-Z) |
| 0 | 0 | X | Write |
| 0 | 1 | 0 | Read |
| 0 | 1 | 1 | Hi-Z |
7. Emulator Interface Requirements
An emulator must expose:
read(address) -> byte
write(address, byte)
Internal storage:
uint8_t ram[32768]
Address masking:
address = address & 0x7FFF
8. Timing Model (Abstracted)
Emulation Levels
| Level | Description |
|---|---|
| Functional | Instantaneous access |
| Cycle-based | Access per CPU cycle |
| Cycle-accurate | Honors enable transitions |
For most systems, functional emulation is sufficient.
9. Power and Data Retention
- SRAM contents persist as long as power is applied
- Emulator shall retain contents until explicitly reset
Reset Behavior
- No automatic clearing on reset
- Memory contents undefined unless initialized
10. Bus Contention and Hi-Z Modeling (Optional)
Optional advanced behavior:
- Track when SRAM drives the data bus
- Detect illegal simultaneous writes
Most emulators may ignore Hi-Z state.
11. Error Conditions
| Condition | Emulator Response |
|---|---|
| Out-of-range address | Mask or ignore |
| Read when disabled | Return last bus value or 0xFF |
| Write when disabled | Ignore write |
12. Integration with 6502 Emulator
CPU memory access
address decode
if in SRAM range:
AS6C62256.read/write
- SRAM access is typically single-cycle
- No wait states required
13. Testing and Validation
Basic Tests
- Write/read patterns
- Boundary addresses ($0000, $7FFF)
- Randomized memory tests
Validation Checklist
- Writes persist
- Reads return correct values
- Address wrapping correct
14. Common Mistakes
| Mistake | Result |
|---|---|
| Clearing RAM on reset | Breaks software assumptions |
| Wrong address mask | Mirrored memory errors |
| Treating as ROM | Writes ignored |
15. Reference Links
Document Scope: Software emulation of AS6C62256 SRAM Audience: Emulator developers, retro SBC designers Status: Stable technical reference