|Red Hat Enterprise Linux 3: Using as, the Gnu Assembler|
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The opcode/operand syntax follows the ESA/390 Principles of Operation manual; assembler directives and general syntax are loosely based on the prevailing AT&T/SVR4/ELF/Solaris style notation. HLASM-style directives are not supported for the most part, with the exception of those described herein.
A leading dot in front of directives is optional, and the case of directives is ignored; thus for example, .using and USING have the same effect.
A colon may immediately follow a label definition. This is simply for compatibility with how most assembly language programmers write code.
# is the line comment character.
; can be used instead of a newline to separate statements.
Since $ has no special meaning, you may use it in symbol names.
Registers can be given the symbolic names r0..r15, fp0, fp2, fp4, fp6. By using thesse symbolic names, as can detect simple syntax errors. The name rarg or r.arg is a synonym for r11, rtca or r.tca for r12, sp, r.sp, dsa r.dsa for r13, lr or r.lr for r14, rbase or r.base for r3 and rpgt or r.pgt for r4.
* is the current location counter. Unlike . it is always relative to the last USING directive. Note that this means that expressions cannot use multiplication, as any occurrence of * will be interpreted as a location counter.
All labels are relative to the last USING. Thus, branches to a label always imply the use of base+displacement.
Many of the usual forms of address constants / address literals are supported. Thus,
.using *,r3 L r15,=A(some_routine) LM r6,r7,=V(some_longlong_extern) A r1,=F'12' AH r0,=H'42' ME r6,=E'3.1416' MD r6,=D'3.14159265358979' O r6,=XL4'cacad0d0' .ltorg
should all behave as expected: that is, an entry in the literal pool will be created (or reused if it already exists), and the instruction operands will be the displacement into the literal pool using the current base register (as last declared with the .using directive).