Information Technology Reference
In-Depth Information
Bus controller
Read/ Write
R/W
Memory/Isolated
M/IO
Memory
mapped
I/O
Address bus
Microprocessor
Interface
devices
Data bus
Isolated
I/O
Figure 2.17
Access memory mapped and isolated I/O
FFFFFFFFh
(4 GB)
Extended
memory
00FFFFFFh
(16 MB)
Extended
memory
000FFFFFh
(1 MB)
Video graphics
Text display
0009FFFFh
(640 KB)
Application programs
(640 KB)
00000600h
Interrupt vectors
BIOS
Figure 2.18
Typical PC memory map
00000000h
2.3.3 Isolated I/O
Devices are not normally connected directly onto the address and data bus of the computer
because they may use part of the memory that a program uses or they could cause a hardware
fault. On modern PCs only the graphics adaptor is mapped directly into memory, the rest
communicate through a specially reserved area of memory, known as isolated I/O memory.
Isolated I/O uses 16-bit addressing from
0000h
to
FFFFh
, thus up to 64 KB of memory
can be mapped. Figure 2.19 shows an example for a computer in the range from
0000h
to
0064h
and Figure 2.20 shows from
0378h
to
03FFh
. It can be seen from Figure 2.19 that
the keyboard maps into addresses
0060h
and
0064h
, the speaker maps to address
0061h
