awesome-copilot/skills/legacy-circuit-mockups/references/emulator-6C62256.md

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.

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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

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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)

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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.

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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

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5. Read and Write Behavior

Read Cycle (Logical)

Conditions:

• CE# = 0
• OE# = 0
• WE# = 1

Behavior:

D[7:0]  memory[A]

If OE# = 1 or CE# = 1, data bus is high-impedance.

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Write Cycle (Logical)

Conditions:

• CE# = 0
• WE# = 0

Behavior:

memory[A]  D[7:0]

• OE# is ignored during writes
• Write occurs on active WE#

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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

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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

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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.

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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

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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.

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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

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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

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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

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14. Common Mistakes

Mistake	Result
Clearing RAM on reset	Breaks software assumptions
Wrong address mask	Mirrored memory errors
Treating as ROM	Writes ignored

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15. Reference Links

• Alliance Memory AS6C62256 Datasheet
• https://en.wikipedia.org/wiki/Static_random-access_memory

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Document Scope: Software emulation of AS6C62256 SRAM Audience: Emulator developers, retro SBC designers Status: Stable technical reference