neither DROP nor USING affect the register contents.

Once a register has been loaded with the address of a piece

of storage, the using statement can be used to 'map' that

storage against a set of labels defining the layout of that

storage e.g. a DSECT. Then whenever one of those labels is

referenced in the code, in moves etc, the assembler resolves

the relative address as a particular displacement from the

absolute address in the register. The DROP instruction

removes the relationship between the labels and the register

and renders subsequent references to those labels as

unresolvable, giving rise to errors at assembly (compile)time.

Typically the DROP instruction will be used to allow use of

the register for another purpose, e.g. address a different

bit of storage via a using staement on second DSECT without

the risk of corrupting that data via moves referencing the

original DSECT.