Answer Posted / Raj Ratna Singh

1. In an MVS assembly program, the USING directive is typically used after BASR (Base Register) to set the base address for the specified register. If you omit the USING directive before BASR, it won't cause an error, but the base address may not be properly set, making the following instructions potentially incorrect or inefficient.

2. Yes, the USING directive tells the assembler to treat a specific register as a base register with the mentioned base address (in this case, the current location counter). The purpose of BASR R12,R0 is to set the register R12 as the base register with the current location counter as its base address.

3. In your example code, there's no issue with address resolution because you are using LA (Load Address) instructions to load an absolute address into a register, bypassing the need for indirect addressing via the base register. However, it might not be the most efficient way in terms of memory usage or performance, since you could potentially overwrite important data stored in other parts of memory.

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