# Category : Alternate Operating Systems - Quarterdeck DesqView, CP/M, etc

Archive : QWAUG92.ZIP

Filename : HEX.TEC

ID:HX Hexadecimal Numbers and QEMM

Quarterdeck Technical Note #190

by Vernon Balbert

Q: Why are these numbers mixed up with these letters and what do they mean?

A: These numbers and letters are part of a numbering system that is used by

computers.

Computers don't count the way people do. People use decimal, or base 10

for counting and we use the numbers 0-9 giving us 10 different symbols for

counting. Computers, on the other hand, use binary, a number system that uses

just zeros and ones. So, a typical binary number might look like this:

1001011011010010

Kind of confusing, huh? Well, even to programmers, these numbers are

confusing. People just don't work well with binary. Computers do. Binary

numbers can be translated to decimal quite easily, but they just aren't neat.

For instance, if you use an eight digit binary number, you can count up to

255. Well, this has three decimal numbers and multiples of 256 (counting 0)

is just not very neat or easy. However, there is a different way of counting

that is widely used by computer people. This is called HEXADECIMAL.

Hexadecimal is base 16. What this means is that if you were to start counting

from 0, you would continue up to some single digit that would represent 15.

And that's where the letters come from. Hexadecimal numbers go like this:

0 1 2 3 4 5 6 7 8 9 A B C D E F

Well, part of that looks normal, at least until the 9. The A translates

as 10, the B as 11 on up until the F is equal to 15. After F, you tack on a 1

at the beginning of the number just like we do so you would get 10. And, yes,

10 plus 10 equal 20. Since 10 in hexa- decimal is equal to 16 in decimal, it

would be logical to assume that if 16+16=32 then 20 hexadecimal equals 32

decimal. Rest assured, it does. Often, to tell the difference between

hexadecimal and decimal numbers, programmers will use H at the end of the

number to say that it is hexadecimal, i.e. 10H. Now, if you have 8 binary

numbers, you can count either to 256, or to FFH. (Remember, the H stands for

hexadecimal.) This means that you can count in multiples of 4 binary digits

with one hexadecimal digit. For instance:

10010110=96H=150 base 10

Now, you might ask, why use multiples of 4 binary digits? Well, the

answer is really quite simple. Computer people like to group binary numbers

in groups of eight. A hexadecimal number can represent 4 binary digits. Each

number is called a bit and each group of eight bits is a byte. And computer

memory is measured in bytes. Each byte can represent one character such as

the letter "Q" or the number "3". Your hard disk is measured in megabytes, or

millions of bytes. So, hexadecimal is a number system that both computers

and humans can work with easily. Actually, computers don't work with

hexadecimal, but most programs that work with hexadecimal numbers have

routines that translate the hexadecimal numbers to binary for the computer

because it's so easy; simply convert each hex digit into the appropriate 4

binary digits. By the way, hexadecimal is often referred to as "hex" for

short.

Okay, now we have to delve a little into the history of IBM PCs to

understand the way you address memory.

When IBM first came out with the PC, the Intel 8088 processor was used.

The 8088 can access 1 megabyte of memory. However, the chip only has 16 bit

registers. (A register is an internal memory loca- tion.) You can only count

to 65535 with 16 bits. In order to access the 20 bits outside the

microprocessor, an alternate method of ad- dressing memory was developed. So,

addresses had the following format:

ssss:oooo

The first four hex digits (ssss) are the segment address. The last four

(oooo) are the offset address. Together, these can address the entire 1

megabyte space of the 8088. This is called relative addressing. You never

actually say the exact address that you are pointing at, just an address

relative to a different position. For instance, if you have an address such

as 1234:2319, you would take 1234H, add a 0 to the end so it turns into 12340H

and then add 2319H to it to get 14659H.

When the 80286 came out, it had 24 address lines to address more than the

1 megabyte of memory available. (24 address lines lets you address 16

megabytes of memory.) Additionally, the internal registers were enlarged to 24

bits so you didn't have to go through the segment and offset gyrations. So,

you could now use absolute addressing. The address now takes this format:

XXXXXX

Notice now, that there are 6 digits instead of two sets of 4 when an

80286 is in "protected" mode (6 digit mode). DOS, unfortunately, only

understands segmented memory addressing which is in "real" mode. Intel built

in a "real" mode into the 80286 and higher processors so that they would still

understand relative addressing. This difference is VERY important. It is the

main reason that you can't use exTENDed memory (only available in "protected"

mode) to run DOS programs (which require "real" mode).

QEMM, QRAM and DESQview all observe the relative addressing scheme. So,

when you include or exclude areas of memory with QEMM, you use the segment

address. Typically, the only exclusions you will use involve the area above

the 640K program space. This is a 384K area of memory that is normally used

by adapter cards and ROMs in your computer. The address range of the lower

640K is 0000-9FFF. Remem-ber, these are the segment addresses, not the offset

addresses. The area above 640K is the A000-FFFF range.

Here is a translation table so you can convert some common hexadecimal

addresses into decimal and back.

Common way of Decimal Hexadecimal Segment:Offset

saying it

1K 1024 400 0040:0000 or 0000:0400

2K 2048 800 0080:0000 or 0000:0800

4K 4096 1000 0100:0000 or 0000:1000

8K 8192 2000 0200:0000 or 0000:2000

16K 16384 4000 0400:0000 or 0000:4000

32K 32768 8000 0800:0000 or 0000:8000

64K 65536 10000 1000:0000

128K 131072 20000 2000:0000

256K 262144 40000 4000:0000

384K 393612 60000 6000:0000

512K 524288 80000 8000:0000

640K 655350 A0000 A000:0000

704K 720896 B0000 B000:0000

736K 753664 B8000 B800:0000

768K 786432 C0000 C000:0000

832K 851968 D0000 D000:0000

896K 917504 E0000 E000:0000

960K 983040 F0000 F000:0000

1024K 1048576 100000 1MB and beyond cannot

be addressed with the

segment:offset method.

In summary, we see that hexadecimal (or hex, for short) is actually base 16,

decimal is base 10 and binary is base 2. Hex numbers go from 0 to F, decimal

numbers go from 0 to 9 and binary go from 0 to 1. Hex numbers are used to

make it easier for people to work with numbers that the computer uses.

************************************************************************

*This technical note may be copied and distributed freely as long as it*

*is distributed in its entirety and it is not distributed for profit. *

* Copyright (C) 1990-2 by Quarterdeck Office Systems *

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