on power-up like previous Commodore machines. Instead of the single 6510 microprocessor of the C64, the C128 incorporates a two-CPU design. The primary CPU, the 8502, is a slightly improved version of the 6510, capable of being clocked at 2 MHz. The second CPU is a Zilog Z80 which is used to run CP/M software, as well as to initiate operating-mode selection at boot time. The two processors cannot run concurrently, thus the C128 is not a multiprocessing system. The C128's complex architecture, includes four differently accessed kinds of RAM (128 KB main RAM, 16–64 KB VDC video RAM, 2 kNibbles VIC-II Color RAM, 2-kilobyte floppy-drive RAM on C128Ds, 0, 128 or 512 KB REU RAM), two or three CPUs (main: 8502, Z80 for CP/M; the 128D also incorporates a 6502 in the disk drive), and two different video chips (VIC-IIe and VDC) for its various operational modes. Early versions of the C128 occasionally experience temperature-related reliability issues due to the use of an electromagnetic shield over the main circuit board. The shield was equipped with fingers that contacted the tops of the major chips, ostensibly causing the shield to act as a large heat sink. A combination of poor contact between the shield and the chips, the inherently limited heat conductivity of plastic chip packages, as well as the relatively poor thermal conductivity of the shield itself, resulted in overheating and failure in some cases. The SID sound chip is particularly vulnerable in this respect. The most common remedy is to remove the shield, which Commodore had added late in development to comply with FCC radio-frequency regulations. The C128 has three operating modes. C128 Mode (native mode) runs at 1 or 2 MHz with the 8502 CPU and has both 40- and 80-column text modes available. CP/M Mode uses both the Z80 and the 8502, and is able to function in both 40- or 80-column text mode. C64 Mode is nearly 100 percent compatible with the earlier computer. Selection of these modes is implemented via the Z80 chip. The Z80 controls the bus on initial boot-up and checks to see if there is a CP/M disk in the drive, if there are any C64/C128 cartridges present, or if the Commodore key (which serves as the C64-mode selector) is being depressed on boot-up. Based on these conditions, it will switch to the appropriate mode of operation. Modes C128 In 1984,