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Adding Expansion Cards
APRICOT 340 OWNER’S HANDBOOK 6/7
Base memory address
The first megabyte (1024 kilobytes) of the computer’s memory is
divided into 640 Kbytes of conventional memory and 384 Kbytes
of upper memory.
Some expansion cards are fitted with memory of their own, usually
read-only memory (ROM) containing functional extensions to the
computer’s BIOS (basic input/output system) ROM. Some cards also
have random-access memory (RAM).
In order that this memory can be “seen” by the system processor, it
must be mapped somewhere within the computer’s own address
space. By setting the base memory address you specify where the
card’s memory begins within the address space. Typically, an
expansion card’s memory must be mapped onto the addresses
between C8000h and EFFFFh in upper memory.
The card’s documentation should list its possible base memory
addresses. You will also need to know how much memory the card
has, so that you can leave the right gap between this card’s base
address and the next. (Some cards allow you to specify the amount
of memory you want to map, but usually this is fixed.)
Note
Memory addresses are always written in base 16 or hexadecimal notation.
Unlike the ten digits of the decimal system (0-9), hexadecimal uses sixteen
digits (0-9 and A-F, where A=10, B=11, C=12 and so on up to F=15).
Hexadecimal numbers are denoted either by the suffix “h” or by the prefix
“0x”. The final digit of a five-digit memory address is often omitted, so
C8000h may be written as C800h. Since amounts of memory are usually stated
as kilobytes rather than in hexadecimal notation, the following conversion
table may be helpful:
4 Kbytes = 1000h 32 Kbytes = 8000h
8 Kbytes = 2000h 64 Kbytes = 10000h
16 Kbytes = 4000h 128 Kbytes = 20000h