68000 question

ArPharazon

Im teaching myself 68000 from a book and im having some trouble with the Branch Equal to instruction (BEQ) The book says its really a Branch on the Z-bit in the condition code register being set.

In another part of the book it explains this as the source is taken away from the destination, and if the answer is zero , the Z bit is set.

Here's the first few lines of the example.
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ReadAgain MOVE.B $00008000,D0
MOVE.B D0,D1
AND.B #%10000000,D0
BEQ ReadAgain

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This is just a small piece of the code.
But my question is - what does the "BEQ ReadAgain" instruction do?
What is equal to what?

Tom Dunne

BEQ usually compare one register with another.

So, in the code above, an AND function is being performed between the contents of D0 and the value 10000000 (em, or is it the contents of memory location 10000000? I am not 100% sure).

So after the AND function is performed, the Branch if EQual jumps back to ReadAgain if the contents of D0 is the same as the contents of D1.

(It's been a long, long time since I looked at this stuff, so above may not be 100% accurate)

Herbal Deity

Tom Dunne wrote: »

em, or is it the contents of memory location 10000000? I am not 100% sure

Nope. # means immediate value and % means binary format.

OP, if AND.B #%10000000,D0 results in a 0, the Z bit in the CCR is set. When you call BEQ, it checks the Z bit and if set, branches to the label specified.

AndrewMc

Obviously enough, the AND instructions ands #%10000000 and register D0 storing the result in D0. But as a less obvious side-effect, it also apparently sets the Z bit of the condition code register to 1 if the result of the AND was zero. BEQ will, as you said, jump to the given location if Z is 1.

So it seems like this loop will continue until the top bit of D0 is 1. Since D0 came from a fixed memory address, I'm guessing this corresponds to some external device, and we're waiting for some input or status change?

Found this by reading http://www.tigernt.com/onlineDoc/68000.pdf and http://www.vintagecomputermanuals.com/Retro%20Vintage%20Computer%20Sytem%20Manuals/Corvus%20Systems/Concept/Assembler%2068000%20Programmers%20Reference%20Manual.pdf

[edit: late reply, Herbal got there first ]

ArPharazon

Yes that is correct AndrewMc , it was for modeling some device. Thanks everybody for the help. Its "clicked" with me.