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Section
04 Part 04 – The EXT Instruction |
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“I
invented the cordless extension cord.” ~Steven Wright |
Introduction
EXT – sign EXTend a data
register
This
instruction will extend the most significant bit of a byte of the destination operand to word, or the most
significant bit of a word of the destination operand to long-word.
Examples
You
should by now, know about signed numbers and what is positive or negative. To demonstrate the purpose of the EXT
instruction, we’ll start with d0 containing 000000FC.
If we
read d0 as a byte, we get FC. This is negative 04. However, if we read d0 as a word, we get 00FC, which is in fact positive
instead.
The
EXT instruction is designed to extend the polarity from byte to word, or from word to long-word. For example:
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ext.w d0 |
After
this instruction, d0 will now contain 0000FFFC.
So
now, if we read d0 as a word, we get FFFC,
which is now negative 0004. However, once again, if we read d0 as a
long-word, we get 0000FFFC, which is positive. Once again, using the same instruction but .l
for long-word size:
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ext.l d0 |
And
now d0 contains FFFFFFFC. So if we read
d0 as a long-word, we get FFFFFFFC,
which is once again negative 00000004.
We’ll
have a final example with positive numbers.
d0 contains 7FC0E024.
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ext.w d0 |
Positive
is extended as word; d0 now contains 7FC00024, where a word is now positive 0024. A long-word is 7FC00024, which is positive, but it isn’t +24. And so:
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ext.l d0 |
Extending
from word to long-word, now d0 contains 00000024, positive 24 once again.
This
instruction can ONLY be used
on data registers.
The process
This
instruction works by reading the MSB.
For example, if d0 contains FEDCBA14, and we perform:
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ext.w d0 |
What
happens is, the byte 14 of d0 is read, and we’ll take a look at it in binary:
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0001 0100 |
As you
can see, the MSB is 0 (clear). This bit
is extended to the left, so if we take a look at the word of d0 (BA14) in
binary:
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1011 1010 0001 0100 |
After
the ext.w instruction, you’ll be left with:
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0000 0000 0001 0100 |
As you
can see, the MSB has extended and effectively “copied” itself out to a
word. Now d0 contains FEDC0014.
If d0
contains C1208489, and we perform:
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ext.l d0 |
The
word 8489 is read, in binary:
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1000 0100 1000 1001 |
The
MSB is 1 (set). That bit is then
extended to long-word, so looking at d0 as a long-word:
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1100 0001 0010 0000 1000 0100 1000 1001 |
After
ext.l you’ll be left with:
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1111 1111 1111 1111 1000 0100 1000 1001 |
The
MSB of word has extended out to long-word.
Now d0 contains FFFF8489.