HLASM: V1R6 Language Ref

LookaheadSequencelater defined by OPSYN. OPSYN statements are not processed; thus, labels aretreated in accordance with the operation code definitions in effect at the time ofentry to lookahead.symbolsThe conditional assembly instructions AGO and AIF in open code control thesequence in which source statements are processed. Using these instructions it ispossible to branch back to a sequence symbol label and re-use previouslyprocessed statements. Due to operating system restrictions, the primary inputsource can only be read sequentially, and cannot be re-read. Whenever a sequencesymbol in the name field is encountered in open code, the assembler must assumethat all subsequent statements may need to be processed more than once. Theassembler uses the lookahead file to save the statement containing the sequencesymbol label and all subsequent statements as they are read and processed. Anysubsequent AGO or AIF to a previously encountered sequence symbol is resolvedto an offset into the lookahead file and input continues from that point.OpencodeConditional assembly instructions in open code let you:v Select, during conditional assembly, statements or groups of statements from theopen code portion of a source module according to a predetermined set ofconditions. The assembler further processes the selected statements at assemblytime.v Pass local variable information from open code through parameters into macrodefinitions.v Control the computation in and generation of macro definitions using global SETsymbols.v Substitute values into the model statements in the open code of a source moduleand control the sequence of their generation.All the conditional assembly elements and instructions can be specified in opencode.The specifications for the conditional assembly language described in this chapteralso apply in open code. However, the following restrictions apply:To Attributes In Open Code:For ordinary symbols, only references to the type, length, scale, integer,defined, and operation code attributes are allowed.References to the number attribute have no meaning in open code, because&SYSLIST is not allowed in open code, and symbolic parameters have nomeaning in open code.To Conditional Assembly Expressions:Table 55 shows the restrictions for different expression types.Table 55. Restrictions on coding expressions in open codeExpression Must not containArithmetic(SETA)v &SYSLISTv Symbolic parametersv Any attribute references to symbolic parameters, or system variablesymbols with local scope338 HLASM: V1R6 Language Ref