360 Assembly/360 Instructions/DROP

The DROP pseudo-instruction is used to inform the assembler that a register previously declared in a USING pseudo-instruction is no longer available for use as a base register in instructions, and should not be used for that purpose. The assembler will revert to using any remaining register previously declared in a USING statement that has the smallest displacement from the target address. The format for the DROP pseudo-instruction is: DROP register1[,register2[,...]] Register1, register2 etc. can be any of the general purpose registers 1 through 15 which was previously declared in a USING pseudo-instruction. As register 0 is effectively unavailable for use as a base register, it is virtually never used in a USING pseudo-instruction, and so would not be used in a DROP pseudo-instruction Here is an example of an inline subroutine which sets up a temporary base register for the subroutine. As the above example stated, the USING pseudo-instruction tells the assembler that a specific register contains the specified address, while the DROP pseudo-instruction tells the assembler that register can no longer be used, until it is specified in another USING pseudo-instruction.