Hardware specification

The lychee follows the CPU and quite a bit of the hardware architecture of a GameBoy. It has been built to be quite bare bones and has very limited functionality.

To read the full specification as a list read the specs document

Contents

• Hardware specification

  • Contents

  • Display

    • Predefined Colours

      • Colour Guide

  • CPU

    • Registers

    • Flags

      • Examples

    • Instruction set

      • Assembly

  • Memory

    • Character display memory

  • Audio

  • Input

Display

To keep things simple it has a simple text based graphics output. No Bitmaps or tiles.

It consists of 65x45 character display (2880 characters in total). Each character supports 4 bit colour, 16 predefined colours. Each character supports a foreground colour and background colour, represented by a byte, with the upper 4 bits representing the background and the lower 4 bits representing the foreground. The character set is the default C64 character set.

Predefined Colours

The predefined colours are set individually by each display or emulator. The general colour approximation is based on the pico8's palette.

Colour Guide

pico8's colour palette

• $00: Black

• $01: Navy Blue

• $02: Deep Violet

• $03: Dark Green

• $04: Brown

• $05: Dark Grey

• $06: Light Grey

• $07: White

• $08: Red

• $09: Orange

• $0A: Yellow

• $0B: Green

• $0C: Sky Blue

• $0D: Lavender/ Indigo

• $0E: Hot Pink

• $0F: Baby Pink

CPU

The CPU heavily borrows many of the instruction set and behaviours from a GameBoy. It uses an 8 bit instruction set and a 16 bit address bus.

However the CPU is clocked at only 100MHz (over and under clock-able). The CPU also contains a fixed 60Hz timer, that can be used to timed operations.

Registers

• a: Accumulator

• f: Flags (not-writable)

• b: General purpose registers

• c: General purpose registers

• d: General purpose registers

• e: General purpose registers

• h: General purpose registers

• l: General purpose registers

• pc: Program counter (not-writable)

• sp: Stack pointer (not-writable)

The h and l registers are quite often merged to create a 16 bit register used for memory addresses or 16 bit math; denoted by hl or (hl) to point to an address in memory that is stored in hl.

Flags

These flags are set when an operation results in these.

• c: carry

• z: zero

• n: subtraction

• h: half carry

Examples

• If a math operation overflows then c is set.

• If a math operation returns 0 then z is set.

Instruction set

The CPU uses a heavily inspired version of the GameBoy instruction set. This version include some hardware specific instruction.

Assembly

Lychee Assembly, the custom assembly dialect. This version include some syntax specifics. And an official assembler is available.

Memory

Lychee uses memory-mapped memory and I/O. This means all that the ROM, Work RAM, Character display memory and I/O all live under the same memory address range.

Lychee uses a 16 bit address bus, thus 65536 possible address locations. This is then split between the different parts of the machine.

1. ($0000-$7FFF) 32 kb: ROM

2. ($8000-$967F) 5760 bytes: Character display memory

3. ($9680-$977F) 255 bytes: Flags & I/O

4. ($9780-$9999) 538 bytes: Reserved stack ram

5. ($A000 - $FFFF) 24 kb: Work RAM

Character display memory

Between $8001-$9680 is where all the character data is stored. For each address that stores the character, the adjacent memory address is the character's colour data.

As the address goes up we move along the from left to right, once we reach the end of a line we loop back to the left of the next line.

Audio

Not Planned

Input

You get a full ascii keyboard that maps to the C64 character set and will return that.