HLASM Language Reference

USING Instruction
Base Registers for Absolute Addresses
Absolute addresses used in a source module must also be made addressable.
Absolute addresses require a base register other than the base register assigned to
relocatable addresses (as described above).
However, the assembler does not need a USING instruction to convert absolute
implicit addresses in the range 0 through 4095 to their explicit form. The assembler
uses register 0 as a base register. Displacements are computed from the base
address 0, because the assembler assumes that a base or index of 0 implies that a
zero quantity is to be used in forming the address, regardless of the contents of
register 0. The USING domain for this automatic base register assignment is the
entire source module.
| If a register is specified with base address zero, the assembler will use it in
| preference to the default use of register zero. For example:
| USING 3,
| LA 7,5
| generates the instruction X'41703005'; in the absence of the USING statement, the
| generated instruction would be X'41700005'.
For absolute implicit addresses greater than 4095 and in the absence of
long-displacement instructions, a USING instruction must be specified according to
the following:
With a base address representing an absolute expression
With a base register that has not been assigned by a USING instruction in
which a relocatable base address is specified
This base register must be loaded with the base address specified.
Ordinary USING Instruction
The ordinary USING instruction format specifies a base address and one or more
base registers.
Ordinary USING
►►──┬─────────────────┬──USING──┬─base─────────────┬────────────────►
└─sequence_symbol─┘
└─(base─┬────── ┬─)
─┘
└─,end─┘
┌
──────────────┐
►─── ▼ ,base_register┴────────────────────────────────────────────────►◄
sequence_symbol
is a sequence symbol.
base
specifies a base address, which can be a relocatable or an absolute
expression. The value of the expression must lie between 0 and 2 31 −1.
end
specifies the end address, which can be a relocatable or an absolute
expression. The value of the expression must lie between 0 and 2 31 −1. The
end address may exceed the (base address + default range) without error. The
220 HLASM V1R5 Language Reference