A d d i n g E x p a n s i o n C a r d s

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

 

 

 

 

APRICOT 340 OWNER’S HANDBOOK 6/7