Exclusive Or the hard way.

I thought following yesterday's post you might want to see what exclusive or on a TMS1000 is like. This is it :)

(It isn't the most incomprehensible thing I've ever written. That goes to my multi-way scrolling and sprite engine for the TI83 calculator - about half the code was self modifying just to make it run fast enough)

ExclusiveOr:
        tcy     XORResult           ; clear XOR Result
        tcmiy   0                           
        tcy     XORCounter          ; set counter to 4
        tcmiy   4
ExclusiveOrLoop:
        tcy     XORResult           ; shift the result one bit left.
        tma
        amaac
        tam                         ; store it back.

        tcy     XORNibble1          ; read Nibble 1
        tma 
        tcy     XORNibble2          ; point at Nibble 2
        tbit1   3                   ; if bit 3 of Nibble 2 is set
        br      XOBit3Set           ; add 8 to A, eg xor of MSB of Nibble
        br      XOBit3Unchanged     ; because add = exor with no carry in
XOBit3Set:                          ; no carry in is why we cant just add
        a8aac
XOBit3Unchanged:                    ; bit 3 of A is XOR bit 3 of N1,N2.
        alec    7                   ; if it is set, set LSB of result
        br      XONextBit
        tcy     XORResult           ; this bit will be shifted into place
        sbit    0
XONextBit:
        tcy     XORNibble1          ; shift nibble 1 left
        tma
        amaac
        tam
        tcy     XORNibble2          ; shift nibble 2 left
        tma
        amaac
        tam

        tcy     XORCounter          ; Decrement XOR Counter
        dman
        tam                         ; write back
        mnez                        ; check counter not zero
        br      ExclusiveOrLoop     ; and keep going till all four bits.
        tcy     XORResult           ; Load the result into A
        tma                                 
        retn

Kevin Savetz's book.

As a slightly relevant aside, I did get a prize in the last RC, kindly donated by Kevin Savetz (I'm guessing this is him :) )

If you ever want an entertaining read and you are a 70s/80s computer geek, it's highly recommended.

Actually a lot of the book could be about me, (if you cross out where it says "Atari" and replace it with "Sharp MZ-80K and BBC Micro.", I never owned any Atari kit before an Atari ST.