The Compare Halfword instruction is used to compare a binary fullword in a register, Operand 1, with a binary halfword in memory, Operand 2. The operands are compared as 2’s complement signed binary integers. For purposes of comparison, the halfword is “sign-extended” to a fullword before the comparison occurs. This extension occurs internally and is temporary. The instruction sets the condition code to indicate how Operand 1 compares to Operand 2:

Condition Code Meaning Test With

0 Operand 1 = Operand 2 BE, BZ

1 Operand 1 < Operand 2 BL, BM

2 Operand 1 > Operand 2 BH, BP

CH R9,AFIELD

The contents of the halfword “AFIELD”, x012C’, is signed extended to a fullword, x’0000012C’, and is compared to the contents of register 9 which contains x’000045FF’. Since the contents of the register (Operand 1) is greater than the value than the extended halfword (Operand 2), the condition code is set to “High”. The condition code in the diagram above is specified using 2 binary digits. After comparison, the condition code is set to a binary 10 which is 2 in decimal - a “High” condition.

Since CH is an RX instruction, an index register may be coded as part of operand 2 (see Explicit Addressing).

Some Unrelated Compare Halfwords

R4 = X’FFFFFFD5’ -43 IN 2’S COMPLEMENT

R5 = X’00000028’ +40 IN 2’S COMPLEMENT

R6 = X’00000004’ +4 IN 2’S COMPLEMENT

DOG DC H’40’

CAT DC H’-30’

PIG DC H’14’

GOAT DC H’3’

CH R4,=H’20’ CONDITION CODE = LOW

CH R4,=H’-50’ CONDITION CODE = HIGH

CH R5,=H’20’ CONDITION CODE = HIGH

CH R6,=H’4’ CONDITION CODE = EQUAL

CH R5,=H’40’ CONDITION CODE = EQUAL

CH R5,DOG CONDITION CODE = EQUAL

CH R6,DOG(R6) CONDITION CODE = LOW

DOG(R6) IS EQUIVALENT TO PIG