Jbyrd: ZBC wiki page from What_is_Zero_Board_Computer.wiki

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ZBC wiki page from What_is_Zero_Board_Computer.wiki

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= What is Zero Board Computer? =

The '''Zero Board Computer''' ('''ZBC''') is a hardware and system specification designed to provide a minimal, standardized testing environment for CPU architectures. ZBC is not tied to any specific implementation—it can be realized in physical hardware (FPGA, ASIC) or virtual environments (emulators like MAME).

== Overview ==

ZBC defines a complete yet minimal computer system consisting of:
* A CPU of any architecture (8-bit through 128-bit and beyond)
* RAM sized appropriately for the CPU's address space
* An MC6847 video display controller providing 32×16 character text output
* A memory-mapped semihosting peripheral for host I/O services

The specification is '''platform-agnostic''' and '''implementation-independent'''. Whether built in hardware or software, any ZBC-compliant system presents the same interface to programs and behaves consistently across all CPU architectures.

== The Problem ==

Bringing up compilers, debuggers, and runtime libraries on new CPU architectures traditionally requires:
* Complex operating system infrastructure
* Multiple device drivers for I/O, storage, and timing
* Architecture-specific test harnesses
* Physical hardware or complex emulation setups
* Inconsistent testing environments across different platforms

This complexity creates significant barriers to:
* Testing code generation across multiple CPU architectures
* Validating compiler optimizations and runtime behavior
* Developing architecture-agnostic debugging tools
* Educational use of diverse CPU architectures
* Automated testing of CPU emulators

== The Solution ==

ZBC solves these problems by providing a '''minimal, standardized specification''' that:

=== Requires Only Two Device Drivers ===
Programs running on ZBC need drivers for just two devices:
* '''MC6847 text display''' - Simple memory-mapped character output
* '''Semihosting peripheral''' - Memory-mapped I/O for host services

=== Provides Immediate Host Access ===
Through the semihosting interface, programs immediately gain:
* File I/O (open, read, write, close, seek)
* Console I/O (character and string output)
* Timing services (clock, elapsed time)
* System services (execute host commands, get command line)

This means '''printf() works immediately''' without implementing UART drivers, filesystem code, or an operating system.

=== Works on Any CPU Architecture ===
The ZBC specification uses dynamic memory layout calculations that automatically adapt to any address space width. The same specification works equally well on:
* 8-bit CPUs with tiny address spaces
* 16-bit CPUs (6502, Z80, 8086)
* 32-bit CPUs (68000, ARM, i386)
* 64-bit CPUs (x86-64, AArch64, RISC-V)
* Future 128-bit and beyond architectures

=== Guarantees Consistent Behavior ===
Every ZBC implementation, regardless of CPU or platform:
* Presents the same memory layout (scaled to address width)
* Provides the same peripheral interfaces
* Supports the same semihosting syscalls
* Produces deterministic, repeatable results

== System Components ==

=== CPU ===
Any CPU architecture can be used. The CPU connects to the system bus and accesses memory and peripherals using standard load/store operations. No special CPU features are required beyond basic memory access.

=== RAM ===
System RAM is sized automatically based on the CPU's address space width. The dynamic layout algorithm ensures maximum usable RAM while reserving space at high memory for peripherals. For example:
* 16-bit CPUs: ~63KB available RAM
* 32-bit CPUs: ~4GB available RAM
* 64-bit CPUs: Effectively unlimited RAM

=== MC6847 Video Display Generator ===
The MC6847 VDG provides a simple text display:
* 32 columns × 16 rows of characters
* 512 bytes of memory-mapped video RAM
* Standard ASCII character set
* Automatic refresh at ~62Hz (PAL timing)
* Optional VSync interrupt for timing

The MC6847 is a real historical chip used in 1980s home computers like the TRS-80 Color Computer and Dragon 32/64. Its simplicity makes it ideal for bringing up new systems.

=== Semihosting Peripheral ===
A memory-mapped device providing host I/O services:
* 32 bytes of device registers
* RIFF-based protocol for architecture-agnostic communication
* ARM semihosting-compatible syscall interface
* Compatible with standard toolchains (gcc, clang, newlib, picolibc)
* Synchronous (polling) or asynchronous (interrupt-driven) operation

See [[Semihosting Overview]] for complete details.

== Use Cases ==

=== Compiler Bring-Up and Testing ===
* Test code generation for new target architectures
* Validate optimizer behavior across instruction sets
* Debug runtime library implementations
* Verify ABI compliance
* Compare code quality across compilers
* '''No OS required''' - programs run directly on hardware

=== Debugger Development ===
* Test debugger features on multiple architectures
* Controlled, predictable environment for debugging debuggers
* Consistent behavior simplifies test automation
* Standard I/O through semihosting eliminates platform differences

=== Library Porting ===
* Port libc implementations to new architectures
* Test runtime libraries (memory allocation, I/O, timing)
* Validate POSIX compliance
* File I/O and console I/O available immediately through semihosting

=== CPU Emulator Validation ===
* Test CPU emulation accuracy (MAME's primary use case)
* Standardized test harness for hundreds of CPU types
* Automated testing and validation
* Quick verification that emulation is working

=== Educational Platforms ===
* Learn assembly language and systems programming
* Experiment with diverse CPU architectures
* Simple enough for students and beginners
* Immediate visual feedback via text display
* Host I/O services without OS complexity

== Design Philosophy ==

The ZBC specification embodies four core principles:

=== Simplicity ===
Minimal hardware components reduce complexity and implementation effort. Only essential peripherals are included. No unnecessary features or options that complicate the specification.

=== Universality ===
A single specification works across all CPU architectures, from 8-bit to 128-bit and beyond. The dynamic memory layout algorithm automatically adapts to any address space width. Architecture-specific details are isolated and well-defined.

=== Determinism ===
ZBC systems provide predictable, repeatable behavior. Memory layout is calculated algorithmically. Peripheral addresses are fixed relative to address space. Program execution is deterministic for given inputs.

=== Standardization ===
All ZBC implementations present the same interface. Programs written for ZBC work on any compliant implementation. Toolchains need only target the ZBC specification, not individual implementations.

== Comparison with Alternatives ==

=== vs. Full Operating Systems ===
* '''Linux/Unix''': Requires complex kernel, device drivers, filesystem, userspace
* '''Bare Metal Boards''': Inconsistent hardware, limited availability, expensive
* '''ZBC''': Minimal, universal, free, deterministic

=== vs. QEMU System Mode ===
* '''QEMU''': Architecture-specific machine models, heavy resource usage
* '''ZBC''': Single specification for all CPUs, minimal overhead

=== vs. Traditional Semihosting ===
* '''ARM Semihosting''': Requires debugger, uses trap instructions, architecture-specific
* '''ZBC Semihosting''': No debugger needed, memory-mapped I/O, works on any CPU

== Implementation Status ==

=== MAME Implementation ===
MAME (Multiple Arcade Machine Emulator) provides a reference implementation of the ZBC specification supporting hundreds of CPU architectures. The MAME implementation uses C++ templates to generate ZBC systems automatically for all supported CPUs.

See [[MAME ZBC Implementation]] for implementation-specific details.

=== Hardware Implementation ===
The ZBC specification can be implemented in physical hardware using FPGAs or custom ASICs. The specification is designed to be realizable with discrete components or integrated designs.

== Next Steps ==

* [[Getting Started]] - Run your first ZBC system
* [[System Overview]] - Understand the system architecture
* [[Semihosting Overview]] - Learn about host I/O services
* [[Memory Layout and Addressing]] - Understand memory organization
* [[Writing Programs for ZBC]] - Develop software for ZBC

== See Also ==

* [[Design Goals and Use Cases]] - Detailed exploration of ZBC's purpose
* [[Key Concepts]] - Essential terminology and concepts
* [[ZBC Specification]] - Complete technical specification

{{ZBC Navigation}}

[[Category:Overview]]
[[Category:Foundation]]

What is Zero Board Computer - Revision history

Jbyrd: ZBC wiki page from What_is_Zero_Board_Computer.wiki