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The internal state can be held as follows:

A known interface to talk to the simulated memory.

A table mapping said functions onto the opcode map.

Functions to simulate each instruction.

The amount of time that the CPU has run in total.

The amount of time used to execute the last instruction.

A structure for retaining the current state of the registers.

The basic model of the Z80 emulation would therefore require the following components: There are other registers in the Z80, that deal with holding status: the flags register (F), whose operation is discussed below and the stack pointer (SP) which is used alongside the PUSH and POP instructions for basic LIFO handling of values. Each of these can be simulated by a JavaScript function, that operates on an internal model of the registers, and produces effects on an internal model of the memory interface. That table is detailed in the Gameboy Z80 opcode map released on this site. If there are 256 possible values in the first byte of an instruction, that makes for 256 possible instructions in the basic table. Most of the instructions in the Z80 are used to handle values in these registers: loading a value from memory into a register, adding or subtracting values, and so forth. Each of them is one byte, so each one can hold a value from 0 to 255. In addition to the PC, other numbers are held inside the CPU that can be used for calculation, and they're referred to as registers: A, B, C, D, E, H, and L.

An instruction can be anywhere between one and three bytes.

Programs are accessed through the same address bus as normal memory.

The memory interface can address up to 65,536 bytes (a 16-bit address bus).

The Z80 is an 8-bit chip, so all the internal workings operate on one byte at a time.

The CPU in the original GameBoy is a modified Zilog Z80, so the following things are pertinent:

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After an instruction is fetched from memory, the PC is advanced by however many bytes make up the instruction. In order to keep track of where the CPU is within the program, a number is held by the CPU called the Program Counter (PC).

From the CPU's perspective, a loop starts up as soon as the computer is turned on, to fetch an instruction from memory, work out what it says, and execute it. Each instruction takes a short amount of time to run, and they're all run one by one. The traditional model of a computer is a processing unit, which gets told what to do by a program of instructions the program might be accessed with its own special memory, or it might be sitting in the same area as normal memory, depending on the computer.

This article sets out to implement the basis for a GameBoy emulation, by laying the groundwork for emulating each part of the physical machine. The simplest way of checking whether this is viable is, of course, to write such an emulator in JavaScript. With the recent introduction of the tag to HTML, the question arises as to whether a JavaScript program is capable of emulating a system, much like desktop applications are available to emulate the Commodore 64, GameBoy Advance and other gaming consoles.

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However, JavaScript is a full object-oriented programming language, and is used in arenas besides the Web: the Widgets available for recent versions of Windows and Apple's Mac OS are implemented in JavaScript, as is the GUI for the Mozilla application suite. It's often stated that JavaScript is a special-purpose language, designed for use by web sites to enable dynamic interaction.

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The emulator described in this series is available in source form: This is part 1 of an article series on emulation development in JavaScript ten parts are currently available, and others are expected to follow.