check_for_prime.ae

. Checks whether a number is prime. The example value (203) celebrates
. the 203rd anniversary of Ada's 1815 birth in 2018.

N001 203 the number to be checked

N000 0   constant 0 - actually the default
N002 1   constant 1
N003 2   constant 2
N004 3   starting test divisor

. column 10 is used as a scratch register; it must be zero before
. storing into it, so always load using Z10

A write in columns

. if target number is < 4, it's prime
+        add 1 to 3, makes 4
L004
L002
S010
-        subtract 4 from target number
L001
Z010     leave column 10 zero
CF?1     skip if sign changed (target < 4)
CF+9     continue
A write numbers as ##9
+        report the target is prime
L001
L000
P
A write annotation  is prime
A write new line
H

. if target number is divisible by 2, it isn't prime
/
L001
L003
S010     store remainder
-        check for zero
Z010     load remainder, leaving column 10 zeroed
L002     subtract 1
CF?1     skip to print/halt if sign changed (i.e. was zero)
CF+8     continue to check next divisor
A write numbers as ##9
+        print the target number
L001
L000
P
A write annotation  is divisible by 2
A write new line
H

. main loop
/        divide target by test value
L001
L004
S010     store remainder (column 10 must be zero previously)
-        check for zero
Z010     load remainder, leaving column 10 zeroed
L002     subtract 1
CF?1     skip to print/halt if sign changed (i.e. was zero)
CF+15    continue to check next divisor

. divisor found!
A write numbers as ##9
+        print the target number
L001
L000
P
A write annotation  is divisible by
A write numbers as  ##9
+        print the divisor
L004
L000
P
A write new line
H

+        check next possible factor
Z004     load current candidate, clear store
L003     add 2
S004     store result
*        square the candidate
L004
L004
S010
-
L001     load target
Z010     load (candidate squared), leaving column 10 zeroed
CF?1     skip if sign changed (i.e. no divisor found)
CB+39    back to main loop

. no divisor found!
A write numbers as ##9
+        report the target is prime
L001
L000
P
A write annotation  is prime
A write new line
H