GCD Euclid using Subtraction only in MMIX

gcdeuclid.mms

        LOC #1000

Tmp     IS   $252

% Entry point
Main
        SETI   $0, 48
        SETI   $1, 18
        PUSHJ  $0, GcdEuclid
        BN     $1, Error
        JMP    Done


% ----------------------------------------------------
% Compute GCD of two non-negative integers using Euclid's algorithm
% Inputs: $0 = a, $1 = b
% Output: $0 = GCD(a, b), $1 = 0 on success, -1 on error 
% ----------------------------------------------------
GcdEuclid
        BN     $0, GcdErrorEnd  % if first number < 0, error
        BN     $1, GcdErrorEnd  % if second number < 0,
        BZ     $0, AnsB      % if a == 0, done
        JMP    IsAnsA
AnsB
        SET    $0, $1
        JMP    GcdEnd
IsAnsA  BZ     $1, GcdEnd      % if b == 0, done
GcdLoop       
        CMP    $10, $0, $1      % if b == a, done
        BZ     $10, GcdEnd      
        BP     $10, GcdAB       % if a > b, continue
        SET    Tmp, $0          % else if a < b, swap
        SET    $0, $1           % a = b
        SET    $1, Tmp          % b = Tmp
GcdAB   SUB    $0, $0, $1      % a = a - b
        JMP    GcdLoop
GcdErrorEnd
        SETI   $1, -1      % return -1 for error
        POP    2, 0        % return to caller

GcdEnd
        SETI  $1, 0
        POP    2, 0        % return to caller

Done    TRAP   0, Halt, 0

Error   SETI   $255, -1
        TRAP   0, Halt, 0


TestCount IS   $253

Test    SETI  TestCount, 0
        JMP    Test1

Test1   ADDUI  TestCount, TestCount, 1
        SETI   $0, 48
        SETI   $1, 18
        PUSHJ  $0, GcdEuclid
        BN     $1, Error
        CMPI   $10, $0, 6
        BZ     $10, Test2
        JMP    Error

Test2   ADDUI  TestCount, TestCount, 1
        SETI   $0, -1
        SETI   $1, 18
        PUSHJ  $0, GcdEuclid
        SETI   $2, -1
        CMP    $10, $1, $2
        BZ     $10, Test3
        JMP    Error

Test3   ADDUI  TestCount, TestCount, 1
        SETI   $0, 48
        SETI   $1, -5
        PUSHJ  $0, GcdEuclid
        SETI   $2, -1
        CMP    $10, $1, $2
        BZ     $10, Test4
        JMP    Error

Test4   ADDUI  TestCount, TestCount, 1
        SETI   $0, 0
        SETI   $1, 17
        PUSHJ  $0, GcdEuclid
        BN     $1, Error
        CMPI   $10, $0, 17
        BZ     $10, Test5
        JMP    Error        

Test5   ADDUI  TestCount, TestCount, 1
        SETI   $0, 75
        SETI   $1, 0
        PUSHJ  $0, GcdEuclid
        BN     $1, Error
        CMPI   $10, $0, 75
        BZ     $10, Test6
        JMP    Error

Test6   ADDUI  TestCount, TestCount, 1
        SETI   $0, 6
        SETI   $1, 6
        PUSHJ  $0, GcdEuclid
        BN     $1, Error
        CMPI   $10, $0, 6
        BZ     $10, Done
        JMP    Error