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4–50 Alpha Architecture Handbook
To compare two character strings for greater/equal/less:
<initialize R1 to aligned QW address of string1>
<initialize R2 to aligned QW address of string2>
LOOP:
LDQ R3, 0(R1) ; Pick up 8 bytes of string1
LDA R1, 8(R1) ; Increment string1 pointer
LDQ R4, 0(R2) ; Pick up 8 bytes of string2
LDA R2, 8(R2) ; Increment string2 pointer
CMPBGE R31, R3, R6 ; Test for zeros in string1
XOR R3, R4, R5 ; Test for all equal bytes
BNE R6, DONE ; Exit if a zero found
BEQ R5, LOOP ; Loop if all equal
DONE: CMPBGE R31, R5, R5 ;
...
; At this point, R5 can be used to determine the first not-equal
; byte position (if any), and R6 can be used to determine the
; position of the terminating zero in string1 (if any).
To range-check a string of characters in R1 for ‘0’…‘9’:
LDQ R2, lit0s ; Pick up 8 bytes of the character
; BELOW ‘0’ ‘////////’
LDQ R3, lit9s ; Pick up 8 bytes of the character
; ABOVE ‘9’ ‘::::::::’
CMPBGE R2, R1, R4 ; Some R4<i>=1 if character is LT ‘0’
CMPBGE R1, R3, R5 ; Some R5<i>=1 if character is GT ‘9’
BNE R4, ERROR ; Branch if some char too low
BNE R5, ERROR ; Branch if some char too high