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Section
02 Part 05 – The CLR Instruction |
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“Three things are certain: Death, taxes, and lost data. Guess which has occurred.” ~David Dixon, 1998, winning entry of the Haiku Error Messages 21st Challenge by Charlie Varon and Jim Rosenau, sponsored by Salon.com |
Introduction
CLR – CLeaR
an operand
This
instruction will clear the destination operand, setting it all to zero (0).
Examples
This
instruction is straight forward:
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clr.b d0 |
Byte
(.b) was used, so the end byte is cleared (set to 00).
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clr.w d0 |
Word
(.w) was used, so the end word is cleared (set to 0000).
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clr.l d0 |
Long-word
(.l) was used, so the entire long-word of the register is cleared.
You
can perform this instruction on memory locations (directly, or by using an
address register), examples:
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clr.w $00201000 clr.b $00201FFF clr.w (a4) clr.l $1C(a2) |
However,
you cannot use it directly on
an address register:
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clr.l a0 |
Of
course, alternatives exist of clearing the address register, just an example
here:
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clr.l d0 move.l d0,a0 |
Homework
Below
is a list of instructions, and I’d like you to give a go at running through it,
instruction by instruction:
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move.w #$0010,d0 move.w d0,$00000040 move.w d0,d1 add.w d1,d1 add.w d0,d1 sub.w $00000040,d1 swap d1 move.w d0,d1 movea.l #$00000040,a4 add.w (a4),d1 move.w d1,(a4) exg.l d1,d0 swap d0 clr.w d0 |
All
of the data registers will start with 00000000 to begin with. After all of this is processed, what will d0
contain?
The
answer and working out are on the next part, and just like before, be sure to
give it a good go before moving on.