Posted by: Matt Stansberry
In his October mainframe column, columnist Robert Crawford asked what commonly used Assembler instruction can be used to add three numbers together and place the answer in a fourth location. The answer is Load Address (LA).
Below is Crawford’s answer and the next Assembler Quiz.
Most of us have seen LA used to get the address of a label:
But if you look in the Principle of Operations manual you will see the actual format of the instruction is as follows:
R1 is the register to receive the address
X is the index register
B is the base register
O is an offset to be added to the sum of the base and index registers
In other words, the magical thing we call an address is actually just a number computed by adding two registers together with an offset. So, at the end of this instruction sequence:
LA R1,4 R1=4
LA R2,3 R2=3
LA R3,2(R1,R2) R3=2+4+3
Register 3 contains 9
We received 9 correct answers to this question. The first three will receive a Startbucks gift card.
New Assembler Quiz: One more quiz before I run out of ideas. What sequence of three instructions can exchange the values of two registers without using memory or a third register? As before, send your answers to email@example.com and the first three correct answers will win a $5 Starbucks gift card.