IBM A20i 2255, User Manual

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The following charts represent how the hard disk stores different types of information. Address
ranges and byte sizes are approximate.
System memory map
The first 640 KB of system board RAM is mapped starting at address hex 00000000. A 256 byte
area and a 1 KB area of this RAM are reserved for BIOS data. Memory can be mapped
differently if POST detects an error.
I/O address map
The following table lists resource assignments for the I/O address map. Any addresses that are
not shown are reserved.
Table 1. System memory map.
Address range
(decimal)
Address range (hex) Description
0~640 KB (000000 ~ 09FFFF) Main memory
640~768 KB (0A0000 ~ 0BFFFF) VGA memory
768~816 KB (0C0000 ~ CBFFFF) Video BIOS Memory
816~896 KB (0CC000 ~ 0DFFFF) ISA Card BIOS & Buffer Memory
896~960 KB (0E0000 ~ 0EFFFF)
BIOS Extension Memory
Setup and Post Memory
PCI Development BIOS
960~1024 KB (0F0000 ~ 0FFFFF) System BIOS memory
(100000 ~ Upper Limit) Main memory
(Upper Limit ~ 4 GB) PCI memory
Table 2. I/O address map.
Address (hex) Description
000-00F DMA controller-1
0020 – 0021 Interrupt controller 1
0022 – 003F General I/0 locations, available to PCI bus
040-043 System timer
060-060 Keyboard controller 8742
061-061 System Speaker
070-071 CMOS RAM Address and Real Time Clock
081-08F DMA controller-2
0A0-0A1 Interrupt controller-2