<Li> AT bus: The AT motherboard had a 16 - bit data bus and 24 - bit address bus (16 MB) that was backward compatible with PC - style expansion cards (which were 8 - bit data, 20 - bit address). </Li> <Li> Fifteen IRQs and seven DMA channels, expanded from eight IRQs and four DMA channels for the PC (and XT). The doubling of the IRQs was achieved by adding another 8259A . IRQs 8--15 (from the second 8259A) are cascaded through IRQ 2 of the first 8259A, which leaves 15 available instead of 16 . Similarly, the number of DMA channels was increased by adding another 8237A, also in master - slave configuration . DMA channel 4 is reserved for cascading 0--3 leaving seven channels active . Some IRQ and some DMA channels are used on the motherboard and not exposed on the expansion bus . </Li> <Li> 16 MB maximum memory (because of the 24 - bit address bus of the 286), compared to the PC's 640 KB maximum (the remainder of the 8088's 1024 kB addressable memory space was reserved for ROM and video memory). </Li> <Li> Battery backed real - time clock (RTC) on motherboard with 50 bytes CMOS memory available for power - off storage of BIOS parameters . (The basic PC had required either manual setting of its software clock using Time and Date commands, or the addition of an accessory expansion card with real - time clock, to avoid the default 01 - 01 - 80 file date .) Additionally the AT RTC had a 1024 - Hz timer (on IRQ 8), which was a much finer resolution compared to the 18 - Hz RTC used by IBM PC XT (IRQ 0). The AT timer was accessible via INT 70h . The RTC was implemented using a Motorola MC146818 integrated circuit . </Li>

When did ibm introduced the 20286 based pc/at