RAM configured as chip RAM by a simple hardware modification, which involves fitting a later model (8372A) Agnus chip. Likewise, all versions of the A500 can be upgraded to chip RAM by fitting the chip and adding additional memory. The Amiga 500 also sports an unusual feature for a budget machine, socketed chips, which allow easy replacement of defective chips. The CPU can be directly upgraded on the motherboard to a 68010; or to a 68020, 68030, or 68040 via the side expansion slot; or by removing the CPU and plugging a CPU expansion card into the CPU socket (this requires opening the computer and thus voided any remaining warranty). In fact, all the custom chips can be upgraded to the Amiga Enhanced Chip Set (ECS) versions. The plastic case is made of acrylonitrile butadiene styrene, or ABS. ABS degrades with time due to exposure to oxygen, causing a yellowing of the case. Other factors contributing to the degradation and yellowing include heat, shear, and ultraviolet light. The yellowing can be reversed by using an optical brightener, though without stabilizing agents or antioxidants to block oxygen, the yellowing will return. Technical specifications OCS (1.2 & 1.3 models) or ECS (1.3 and 500+ 2.04 models) chipset. ECS revisions of the chipset made PAL/NTSC mode switchable in software. Sound: 4 hardware-mixed channels of 8-bit sound at up to . The hardware channels have independent volumes (65 levels) and sampling rates, and are mixed down to two fully left and fully right stereo outputs. A software controllable low-pass audio filter is also included. 512 KB of chip RAM (150 ns access time). AmigaOS 1.2 or 1.3 (upgradeable up to 3.1.4 if 2 MB of RAM are installed) One 3.5" double-density floppy disk drive is built in, which is completely programmable and thus can read IBM PC disks, standard Amiga disks, and up to with custom formatting (such as Klaus Deppich's diskspare.device). Uses (5 rotations/second) and . Built-in keyboard. A two-button mouse is included. Graphics PAL mode: 320×256, 640×256, 640×512 (interlaced), 704×576 in overscan. NTSC mode: 320×200, 640×200, 640×400 (interlaced), 704×484 in overscan. Graphics can be of arbitrary dimensions, resolution and colour depth, even on the same screen. The Amiga can show multiple resolution modes at the same time, splitting the screen vertically. Planar graphics are used, with up to 5 bitplanes (4 in hires); this allowed 2, 4, 8, 16 and 32 colour