The Load and Test Register instruction copies the contents of the register specified by Operand 2, into the register specified by Operand 1. The contents of Operand 2 are unchanged by this operation. In this respect LTR is equivalent to the LR instruction. The difference between these instructions is that LTR sets the condition code based on the final contents of the Operand 1 register.

Condition Code Meaning Test With

0 (Zero) Operand 1 = 0 BE, BZ

1 (Negative) Operand 1 < 0 BL, BM

2 (Positive) Operand 1 > 0 BH, BP

Consider the instruction below.

LTR R5,R10

The contents of register 10 are copied to register 5, destroying the previous value in register 5. Register 10 is unaffected by the operation. Since the contents of R5 is positive after completion of the operation, the condition code is set to 2. The diagram below illustrates this operation.

Some Unrelated LTR’s

R4 = X’FFFFFFFF’

R5 = X’00000028’

R6 = X’00000004’

R7 = X’00000000’

LTR R4,R5 R4 = X’00000028’ R5 = X’00000028’ Cond. Code = Positive

LTR R5,R4 R5 = X’FFFFFFFF’ R4 = X’FFFFFFFF’ Cond. Code = Negative

LTR R5,R6 R5 = X’00000004’ R6 = X’00000004’ Cond. Code = Positive

LTR R6,R5 R6 = X’00000028’ R5 = X’00000028’ Cond. Code = Positive

LTR R6,R7 R6 = X’00000000’ R7 = X’00000000’ Cond. Code = Zero

LTR R4,R4 R4 = X’FFFFFFFF’ Cond. Code = Negative

1) LTR is commonly used to test the contents of a single register in order to determine if the binary number is the register is positive, negative or zero. For example, the following code illustrates how to test the contents of register 5.

LTR R5,R5 SET THE CONDITION CODE

BM NEGATIVE IS R5 < 0 ?

BP POSITIVE IS R5 > 0?

ZERO EQU *

...

NEGATIVE EQU *

...

POSITIVE EQU *