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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.
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