Memory Maps

Tiny Board

The TINY board uses a flat 64 KiB memory map typical of 65C02-based systems. The first 256 bytes ($0000–$00FF) are Zero Page RAM, followed by the stack at $0100–$01FF. The range $0200–$03FF is reserved for OS use, while $0400–$7EFF serves as general-purpose RAM.

A small I/O region at $7F00–$7FFF includes only $7F04–$7F07, which is connected to a 65C51 ACIA UART for serial communication. The remainder of the I/O space is currently unused but mapped to RAM, meaning it is technically accessible for reads and writes. However, using this area is not recommended, as future hardware revisions may repurpose it. Users may also reconfigure the address decoding logic by updating the ATF22V10 chip to change or expand the I/O mapping as needed.

The upper 32 KiB ($8000–$FFFF) is mapped to ROM, containing system firmware such as a monitor or BASIC interpreter.

Address Range	Size	Description
$0000-$00FF	256 bytes	Zero Page RAM
$0100-$01FF	256 bytes	Stack
$0200-$03FF	512 bytes	Reserved (OS Use)
$0400-$7EFF	~31 KiB	General Purpose RAM
$7F00-$7F03	4 bytes	Unused / Reserved
$7F04-$7F07	4 bytes	ACIA (UART)
$7F08-$7FFF	248 bytes	Unused / Reserved
$8000-$FFFF	32 KiB	ROM

Mini Board

The MINI board features a flexible 64 KiB memory map designed to support both fixed and bank-switched memory regions. The lower memory ($0000–$7EFF) is primarily RAM, with the first 256 bytes ($0000–$00FF) allocated as Zero Page, and the next 256 bytes ($0100–$01FF) serving as the stack. Addresses $0200–$07FF are reserved for OS-level functionality, while the remainder up to $7EFF is available as general-purpose RAM, implemented across fixed banks 0–3.

The region $7F00–$7FFF is dedicated to memory-mapped I/O, divided into four 64-byte blocks using only address lines A6 and A7. The first block ($7F00–$7F3F) connects to a 65C51 ACIA for serial communication, while the second ($7F40–$7F7F) maps to a 6522 VIA. The remaining two blocks ($7F80–$7FBF and $7FC0–$7FFF) are used to write to the ROM and RAM bank registers, respectively.

The upper half of the address space is split between banked and fixed ROM/RAM. The ranges $8000–$9FFF and $A000–$BFFF are bank-switched RAM and ROM windows, supporting 64 banks each. The final 16 KiB ($C000–$FFFF) is mapped to fixed ROM, permanently connected to banks 0 and 1, containing critical system firmware.

Warning

The bank-switched ROM and RAM regions ($A000–$BFFF and $8000–$9FFF) share the same physical memory space as the fixed ROM ($C000–$FFFF) and fixed RAM ($0000–$7EFF), respectively. This means that selecting ROM banks 0–1 or RAM banks 0–3 will expose the same physical memory already mapped in the fixed regions. For example, if RAM bank 0 is selected, writes may unintentionally modify the stack or zero page; selecting ROM bank 0 might interfere with the interrupt vectors or firmware routines.

While overlapping ROM banks are less problematic—since ROM is typically read-only and cannot be written to—it can still cause confusion or redundancy if the same code or data appears in both fixed and banked areas. It is strongly recommended to begin using RAM banks from 4 upwards and ROM banks from 2 upwards to avoid overlapping with fixed memory areas and ensure predictable behavior.

Address Range	Size	Description
$0000-$00FF	256 bytes	Zero Page RAM
$0100-$01FF	256 bytes	Stack
$0200-$07FF	512 bytes	Reserved (used by BCOS)
$0800-$7EFF	30 KiB	General Purpose RAM (Fixed, Banks 0–3)
$7F00-$7F3F	64 bytes	ACIA (UART)
$7F40-$7F7F	64 bytes	VIA
$7F80-$7FBF	64 bytes	ROM Bank Register
$7FC0-$7FFF	64 bytes	RAM Bank Register
$8000-$9FFF	8 KiB	Bank-switched RAM Window (Banks 0–63)
$A000-$BFFF	8 KiB	Bank-switched ROM Window (Banks 0–63)
$C000-$FFFF	16 KiB	Fixed ROM (Banks 0–1 mapped permanently)