Nintendo NES-001 User Guide - Page 9

2 - Central Processing Unit 2.1 2A03 Overview Ricoh produced an NMOS processor based on the 6502, the 2A03. The chip differed from a standard 6502 in that it had the ability to handle sound, serving as pAPU (pseudo-Audio Processing Unit) as well as CPU, and that it lacked a Binary Coded Decimal (BCD) mode which allowed representing each digit using 4 bits. For the purposes of programming, the 2A03 uses the same instruction set as the standard 6502 which is shown in figure 2-1. The 6502 is a little endian processor which means that addresses are stored in memory least significant byte first, for example the address $1234 would be stored in memory as $34 at memory location x and $12 at memory location (x + 1). Figure 2-1. The 6502 processor [28]. 2.2 CPU Memory Map Figure 2-2 shows how the CPU accesses memory using buses. The memory is divided into three parts, ROM inside the cartridges, the CPU's RAM and the I/O registers. The address bus is used to set the address of the required location. The control bus is used to inform the components whether the request is a read or a write. The data bus is used to read or write the byte to the selected address. Note that ROM is read-only and is accessed via a MMC, to allow bank switching to occur. The I/O registers are used to communicate with the other components of the system, the PPU and the control devices. Figure 2-2. Processor diagram. 9