HC 77B Technical Overview
The HC-77B is the first publicly released HomeComp machine.
| Field | Detail |
|---|---|
| Machine | HC-77B |
| Launch context | September 1977 launch system |
| Type | Front-panel single-board microcomputer |
| CPU | MOS Technology 6502 |
| Clock | 1 MHz |
| Primary interface | Front-panel display and keypad |
| Storage | Cassette interface |
| Release status | Publicly released |
The HC-77B launch release establishes the starting point for HomeComp Ecosystem Era 1.
HC-77B system board overview.
| Area | HC-77B Launch Specification |
|---|---|
| CPU | MOS Technology 6502 |
| Clock | 1 MHz |
| Base RAM | 1 KB |
| Planned RAM expansion | Up to 4 KB through the official RAM expansion path |
| ROM | Boot/monitor ROM included with the emulator release |
| Display | 8-digit seven-segment front-panel display |
| Input | Hexadecimal keypad with dedicated monitor keys |
| Cassette | Built-in cassette support in the emulator; blank tapes can be created |
| TTY expansion | Available at launch |
| Keyboard expansion | Available at launch |
| Video expansion | Available at launch |
| Expansion model | External expansion/peripheral support through the HC-77B expansion API |
The HC-77B is a front-panel-first machine.
It is operated through its hexadecimal keypad and 8-digit seven-segment display. The monitor ROM provides the basic control environment: inspecting memory, entering data, viewing registers, starting execution, and using cassette load/save functions.
The HC-77B should not be understood as a BASIC-first home computer. It begins as a monitor-controlled 6502 system, with richer software and hardware capability appearing through later ecosystem releases and expansion hardware.
For machine-level operation, the included HC-77B User Manual PDF is authoritative.
The September 1977 launch release presents the HC-77B as a 1 KB base system. The late-1977 3K RAM Expansion extends the machine to the planned 4 KB configuration.
TTY, keyboard, and video expansions are available at launch. Cassette support is active in the emulator from launch, including the ability to create blank tapes.
The HC-77B uses a conventional 6502-style low-RAM, high-ROM arrangement.
| Address Range | Purpose | Notes |
|---|---|---|
$0000-$03FF |
Base RAM | 1 KB fitted in the launch machine |
$0400-$0FFF |
RAM expansion | Used by the official 3K RAM Expansion |
$1000-$EFFF |
General expansion space | Reserved for expansion ROM, RAM, or peripheral use |
$F000-$F1FF |
Screen RAM | Used when video expansion is fitted |
$F200-$F6FF |
Reserved I/O and expansion space | Video, keyboard, peripheral, and future device space |
$F700-$F7FF |
Base I/O | Core machine I/O region |
$F800-$FFFF |
Monitor ROM | Boot and monitor ROM |
This wiki gives only the high-level memory map. The included user manual and technical documents should be treated as the authoritative released documentation for detailed operation.
The HC-77B front panel provides the base machine interface.
| Component | Description |
|---|---|
| Display | 8-digit seven-segment LED display |
| Main input | Hexadecimal keypad |
| Function keys | Dedicated monitor-control keys |
| Operation style | Direct address/data/register interaction through the monitor ROM |
The front panel remains central even when optional launch expansions are present.
Cassette support is active in the emulator release.
The emulator provides cassette handling through the release's tapes/ folder and supports the creation of blank tapes.
The following expansion paths are available at launch:
| Expansion | Launch Position |
|---|---|
| TTY | Available |
| Keyboard | Available |
| Video | Available |
| RAM expansion | Official 3K RAM Expansion available |
| Cassette media | Supported by the emulator |
The details of using emulator extensions and peripheral examples are covered in the Expansion Bus API section.
The September 1977 emulator release supports the base HC-77B launch environment and includes the boot ROM required to start the machine.
| Feature | Status |
|---|---|
| HC-77B base machine | Supported |
| Boot ROM | Included |
| Front-panel operation | Supported |
| Cassette handling | Supported |
| Blank tape creation | Supported |
| TTY expansion | Available |
| Keyboard expansion | Available |
| Video expansion | Available |
The emulator release includes the authoritative HC-77B machine documentation in PDF form.
Useful repository files:
- HC-77B Product Sheet PDF
- HC-77B Product Sheet Image
- HC-77B Technical Specifications
- 3K RAM Expantion with NIM Cassette Bundle
Use the included documentation for detailed machine operation, worked examples, and front-panel monitor behaviour. This wiki page is intended only as a public technical overview.