Editing IBM PCjr (section)

== Architecture ==

The PCjr is based on an '''Intel 8088''' running at '''4.77 MHz''', the same CPU clock as the 5150 and 5160. It does '''not''' have an Intel 8237 DMA controller; all I/O including floppy operations is done by PIO. As a result, keystrokes can be lost while the floppy drive is being accessed (the keyboard receiver also drops scan codes during disk operation, which is a known PCjr limitation).

Key chips on the motherboard include:
* '''Intel 8088''' &mdash; CPU (no second-source NEC V20 was officially endorsed for the PCjr).
* '''Intel 8259A''' &mdash; programmable interrupt controller.
* '''Intel 8253''' &mdash; programmable interval timer.
* '''Intel 8255A''' &mdash; programmable peripheral interface.
* '''Motorola MC6845''' &mdash; CRT controller, embedded on the motherboard rather than on a video card.
* '''Texas Instruments SN76496''' &mdash; sound chip with three square-wave tone generators plus a noise channel. The same chip used on the Tandy 1000, which is why the chip and its register layout are commonly called "PCjr/Tandy sound."

The PCjr motherboard does not implement memory parity, in order to save chip count and cost. Bit errors in DRAM are not reported.