robzgman
asked on
Little Man Computer Programming
Programming a Fibonacci algorithm, I'm having issues with the concept of where I can assign labels (e.g can I do it for entire sections or just individual lines. How branching works?
What it needs to do:
Prompt user for a number
Input Test number
Set First = 1
Set Second = 1
Set Counter = 2
While Test is greater than Second
Next = Second + First
First = Second
Second = Next
Counter = Counter + 1
If Test is equal to Second
Display Counter
Else
Display zero
What it needs to do:
Prompt user for a number
Input Test number
Set First = 1
Set Second = 1
Set Counter = 2
While Test is greater than Second
Next = Second + First
First = Second
Second = Next
Counter = Counter + 1
If Test is equal to Second
Display Counter
Else
Display zero
User Input INP
Store Input STA TEST
Start + Load Second START LDA SECOND
Subtract Test SUB TEST
Initiate Loop BRP GREATER
Initiate Loop BRZ EQUAL
Initiate Loop BRZ END
Define + Load second GREATER LDA SECOND
Add to First ADD FIRST
Save as Next STA NEXT
Load Second LDA SECOND
Save as First STA FIRST
Load Next LDA NEXT
Save as Second STA SECOND
Load Counter LDA COUNTER
Add to Value ADD INCREM
Save as Counter STA COUNTER
Close Loop BRA START
Define + Load Second EQUAL LDA COUNTER
Output OUT
Stop HLT
Stop END HLT
Define Test = 0 TEST DAT 000
Define First = 1 FIRST DAT 001
Define Next = 0 NEXT DAT 000
Define Second = 1 SECOND DAT 001
Set Counter = 2 COUNTER DAT 002
Set Increment 1 INCREM DAT 001Labels are placeholders for addresses. They can be placed in front of any command. For example, your 'SECOND DAT 001' is an example of a label. It just gives you a way to refer to addresses more clearly. They aren't actual instructions.
Quickly looking at your code, you may want to change the following:
you want to test TEST - SECOND, not SECOND - TEST. So load TEST first, then subtract SECOND, then BRP.
Your call to 'BRZ END' will never take the brance, since the 'BRZ EQUAL' above it will never return. So just remove it.
Otherwise, I think (quick glance) your code is quite good.
for more info on labels, and branching, see
http://en.wikipedia.org/wiki/Little_man_computer
ASKER
It has no errors in compilation. However it just continues to run indefinitely (hundreds of lines) even with a small input of 8 (should return as final '6')
So I'm assuming one of the loop backs isn't working / properly
So I'm assuming one of the loop backs isn't working / properly
User Input INP
Store Input STA TEST
Start + Load Second START LDA TEST
Subtract Test SUB SECOND
Initiate Loop BRP GREATER
Initiate Loop BRZ EQUAL
Define + Load second GREATER LDA SECOND
Add to First ADD FIRST
Save as Next STA NEXT
Load Second LDA SECOND
Save as First STA FIRST
Load Next LDA NEXT
Save as Second STA SECOND
Load Counter LDA COUNTER
Add to Value ADD INCREM
Save as Counter STA COUNTER
Close Loop BRA START
Define + Load Second EQUAL LDA COUNTER
Output OUT
Stop HLT
Stop END HLT
Define Test = 0 TEST DAT 000
Define First = 1 FIRST DAT 001
Define Next = 0 NEXT DAT 000
Define Second = 1 SECOND DAT 001
Set Counter = 2 COUNTER DAT 002
Set Increment 1 INCREM DAT 001The branching is wrong. BRP will branch if >= 0. so you'll never get to the BRZ EQUAL statement.
you probably want something like this instead
BRZ EQUAL
BRP GREATER
BRA END
So if it exits w/o printing something that means it wasn't a FIBONACCI number. I don't know if that's what you want it to do or not.
For example, 1, 2, 3, 5, 8, 13,. ... would print the FIB(x), others would just exit w/o printing
You can run this at:
http://www.atkinson.yorku.ca/~sychen/research/LMC/LittleMan.html
It's slow... but does get the job done...
INP
STA TEST
START LDA TEST
SUB SECOND
BRZ EQUAL
BRP GREATER
BRA END
GREATER LDA SECOND
ADD FIRST
STA NEXT
LDA SECOND
STA FIRST
LDA NEXT
STA SECOND
LDA COUNTER
ADD INCREM
STA COUNTER
BRA START
EQUAL LDA COUNTER
OUT
HLT
END HLT
TEST DAT 000
FIRST DAT 001
NEXT DAT 000
SECOND DAT 001
COUNTER DAT 002
INCREM DAT 001ASKER
Arg, nearly got it - its now working out where the number is a fibonacci number, however if its not a fibonacci number, it just runs out and ends up with a random number - e.g -4
I want it to just display 0 if not a fib number
I want it to just display 0 if not a fib number
ASKER CERTIFIED SOLUTION
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A label is for a single instruction. So, when you jump to the label, the next instruction that is executed is the one where the label was added.
Of course, this means that execution continues with the instruction after that, etc., so the whole "section" of instructions following the label will be executed. In that sense, you could say that the label refers to the whole section (but only for discussion purposes).
>> How branching works?
There are basically three branch instructions :
* BRA (unconditional branch) : sets the PC (program counter) to the given address. The next instruction that is executed is the instruction at that address.
* BRZ (branch if zero) : if the accumulator currently contains the value 0, then the PC is set to the given address. In that case, the next instruction that is executed is the instruction at that address.
* BRP (branch if positive) : if the accumulator currently contains a value >= 0, then the PC is set to the given address. In that case, the next instruction that is executed is the instruction at that address.
If the branch is not taken, the PC is incremented (asusual), and the next instruction (immediately after the branch instruction) is executed.