HLASM Language Reference

USING Instruction
end address must be greater than the base and must have the same
relocatability attributes.
The resolvable range of a USING with an 'end' operand is
base,MIN(495,end-1)
Thus USING base,reg is equivalent to USING (base,base+496),reg.
base_register
is an absolute expression whose value represents general registers 0 through
15.
The default range is 4096 per base register.
The assembler assumes that the base register denoted by the first base_register
operand contains the base address base at execution time. If present, the
subsequent base_register operands represent registers that the assembler
assumes contain the address values base+4096, base+8192, and so forth.
For example:
USING
BASE,9,1,11
has the logical equivalent of:
USING
BASE,9
USING
BASE+496,1
USING
BASE+8192,11
In another example, the following statement:
USING
,12,13
tells the assembler to assume that the current value of the location counter is in
general register 12 at execution time, and that the current value of the location
counter, incremented by 4096, is in general register 13 at execution time.
Computing Displacement: If you change the value in a base register being used,
and want the assembler to compute displacements from this value, you must tell
the assembler the new value by means of another USING statement. In the
following sequence, the assembler first assumes that the value of ALPHA is in
register 9. The second statement then causes the assembler to assume that
ALPHA+1 is the value in register 9.
USING
ALPHA,9
.
.
USING
ALPHA+1,9
Using General Register Zero: You can refer to the first 4096 bytes of storage
using general register 0, subject to the following conditions:
The value of operand base must be either absolute or relocatable zero.
Register 0 must be specified as the first base_register operand.
The assembler assumes that register 0 contains zero. Therefore, regardless of the
value of operand base, it calculates displacements as if operand base were
absolute or relocatable zero. The assembler also assumes that subsequent
registers specified in the same USING statement contain 4096, 8192, etc.
Chapter 5. Assembler Instruction Statements 221