Hardware Reference
In-Depth Information
Photo 1.12 Award BIOS ROM (note the real-time clock crystal adjacent to
the chip)
PC memory allocation
The allocation of memory space within a PC can he usefully illustrated by
means of a memory map. An 8086 microprocessor can address any one of 048
576 different memory locations with its 20 address lines. It thus has a memory
which ranges from 00000 (the lowest address) to FFFFF (the highest address).
We can illustrate the use of memory using a diagram known as a 'memory map'.
Figure 1.19 shows a memory map for the first megabyte of PC memory.
BIOS data area
The memory region starting at address 0400H (see Figure 1.19) contains data
that is maintained by the BIOS. A number of memory locations within this space
can provide useful information about the current state of a PC. You can easily
display the contents of these memory locations (summarized in Table 1.13)
using the MS-DOS DEBUG utility (see page 136) or using a short routine
written in QuickBASIC.
As an example of the first method, the following DEBUG command can be
used to display the contents of 10 bytes of RAM starting at memory location
0410:
D0:0410 L 0A
A rather more user-friendly method of displaying the contents of RAM is shown
in the following QuickBASIC code fragment:
DEF SEG = 0
CLS
INPUT "Start address (in hex) "; address$
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