68HC11 Microcontroller

[stewartraymond]

Question:

Accumulator A contains $FF, and the C flag is cleared. If a DECA instruction is executed, the Z flag will be ____________________.

[zoltan-7]

On Thu, 26 Mar 2009, stewartraymond wrote:

> Question:
>
> Accumulator A contains $FF, and the C flag is cleared. If a DECA
> instruction is executed, the Z flag will be ____________________.


Cleared. DECA subtracts 1 from accumulator A and sets the Z flag if the
result is 0, clears it otherwise. ff - 1 is fe, a non-zero value
therefore the Z flag will be cleared. The value of the C flag before the
instruction is irrelevant and the instruction will not change it.


Zoltan

[John Hartman (NoICE)]

At 10:24 PM 3/25/09, you wrote:
>Question:
>Accumulator A contains $FF, and the C flag is cleared. If a DECA
>instruction is executed, the Z flag will be ____________________.

Worth 5 points on your mid-term exam?

You might consider looking in the processor databook.  You will need
to do so many times in your engineering career.
You might consider using a debugger and single-stepping a DECA to see
what happens.  You will need to do so many times in your engineering
career.  Especially when it turns out that the databook is wrong...


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