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2.5. OTHER CONDITIONAL BRANCHES 23

DECB

BRA KINGS

CABBAGES

LDAA #$0A

JSR OUTA

SWI

2.5 Other Conditional Branches

2.5.1 Signed and unsigned numbers

So far we have only considered comparing two values and branching if they are

equal (BEQ) or not equal (BNE). Designers of HC11 have provided several other

branches. When comparing two numbers, it is important to distinguish between

signed and unsigned quantities. Note that the internal hardware does not deal

with numbers per se bit with binary patterns which are interpreted as numbers.

Conversely, numbers are represented by bit patterns and it up to you to chose the

appropriate interpretation. For example, using binary representations, an 8-bit

quantity can represent any number from 0 to 2

− 1 or 0 to 255. This is the

unsigned representation. What happens if we use an 8-bit counter and starting

at zero and incrementing it by one? The value will go from one to two to three

etc. all the way to 255 and then roll over to zero and start all over again. This

is the unsigned interpretation. Now suppose you start at zero and decrement

by one. The value will go from zero to 255 to 254 etc. But starting from zero

decrementing by one you get -1. Thus 255 really represents -1. This is the signed

interpretation.

This dual interpretation is quite natural and does not pose any

problems except when performing comparisons. In the signed interpretations,

numbers 0 to 127 are non-negative, and numbers 128 to 255 are negative, with

128 representing -128, 129 representing -127 ... and 255 representing -1. Note

that the negative numbers come after the positive numbers thus when comparing

two numbers of the opposite sign, the sense of inequality are reversed. Keeping

this in mind, HC11 hardware designers have two sets of branch conditions, one

for signed numbers (where special attention is given if the two numbers that are

This dual interpretation is something I use often. It is easy to think of the clock, and say the

minute setting. Starting at zero and incrementing by one, I get one, two, · · · to 59 and back to

zero. When I set the alarm on this clock I cannot go back, only forward. So, if I want to decrease

the minute value by 5, I have to advance it by 55. Thus adding 55 is same as subtracting 5, or

in terms of arithmetic 55 is has the dual interpretation of -5.

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Motorola Axiom CMD11E1 Uživatelská příručka Strana 23