UpperCase:
;input: A0 = pointer to the string's base address.
;alters the string in-place.

MOVE.B (A0),D0      ;load a letter
BEQ .Terminated     ;we've reached the null terminator.

CMP.B #'a',D0       ;compare to ascii code for a
BCS .overhead       ;if less than a, keep looping.

CMP.B #'z',D0       ;compare to ascii code for z
BHI .overhead       ;if greater than z, keep looping

AND.B #%1101111,D0  ;this "magic constant" turns lower case to upper case, since they're always 32 apart.
.overhead:
MOVE.B D0,(A0)+     ;store the letter back and increment the pointer.
                    ;If this isn't an alphabetical character, D0 won't change and this store won't affect the string at all.
                    ;If it was a letter, it will have been changed to upper case before storing back.

BRA UpperCase       ;next letter

.Terminated:
RTS
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
LowerCase:
MOVE.B (A0),D0      ;load a letter
BEQ .Terminated     ;we've reached the null terminator.

CMP.B #'A',D0       ;compare to ascii code for A
BCS .overhead       ;if less than A, keep looping.

CMP.B #'Z',D0       ;compare to ascii code for Z
BHI .overhead      ;if greater than Z, keep looping

OR.B #%00100000,D0  ;this "magic constant" turns upper case to lower case, since they're always 32 apart.
.overhead:
MOVE.B D0,(A0)+     ;store the result and get ready to read the next letter.
BRA LowerCase       ;next letter
.Terminated:
RTS
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
ToggleCase:
MOVE.B (A0),D0     ;load a letter and inc the pointer to the next letter
BEQ .Terminated     ;we've reached the null terminator.

MOVE.B D0,D1        ;copy the letter
AND.B #%11011111    ;convert the copy to upper case so we can check it.

CMP.B #'A',D1       ;compare to ascii code for A
BCS overhead        ;if less than A, keep looping.

CMP.B #'Z',D1       ;compare to ascii code for Z
BHI overhead        ;if greater than Z, keep looping

EOR.B #%00100000,D0 ;swaps the case of the letter
overhead:
MOVE.B D0,(A0)+     ;store the result
BRA ToggleCase      ;next letter
.Terminated:
RTS