OK - just work it through
Fist instruction inputs a number - you're told that number is going to be 23. So our processor's accumulator has a value of 23
Next instruction stores the number in the accumulator (23) in memory location 20 - so, write that on a bit of paper...
A 23
20 23
Next instruction inputs a number
Next stores that in location 21 (you're told the number will be 20, so write that down on your bit of paper
A 20
20 23
21 20
Next instruction gets another number
Next instruction stores that in 22
Which you need to write down (you're told the number is 15)
A 15
20 23
21 20
22 15
Now you subtract the contents of 21 - here's the only tricky bit - you are subtracting 21 from the value in the Accumulator - i.e. the processor memory into which the number was input - which in our case is the value 15 (we know because we wrote it down!)
So - 15 - (the value at memory location 21)
which is 15 - 20
which is -5
A -5
20 23
21 20
22 15
Next instruction says that, if the result is positive to go to memory location 10 - but it is not so we just carry on...
Next instruction says load the value at 21 - which is still 20 (we never stored the result of the subtraction)
A 20
20 23
21 20
22 15
Next instruction is an unconditional branch to 11 - so we branch to the instruction at 11 (subtract 20)
This instruction subtracts the contents of 20 (which is currently 23) from the value in the accumulator (which is currently 20)
20 - 23 = -3
A -3
20 23
21 20
22 15
Now the output instruction is puzzling - but I assume it just outputs the value in the accumulator somewhere - in which case we will output -3
Hopefully you can follow along with the other values you have been given.
(Excuse formatting problems - the web site isn't being very responsive so I can't see what it will look like!)
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