HLASM: V1R6 Language Ref

Decimal instructionsDecimal data may be represented in either zoned or packed format. In the zonedformat, the rightmost four bits of a byte are called the numeric bits and normallyconsist of a code representing a decimal digit. The leftmost four bits of a byte arecalled the zone bits, except for the rightmost byte of a decimal operand, wherethese bits may be treated either as a zone or as a sign.In the packed format, each byte contains two decimal digits, except for the rightmostbyte, which contains a sign to the right of a decimal digit.Decimal instructions treat all numbers as integers. For example, 3.14, 31.4, and 314are all processed as 314. You must keep track of the decimal point yourself. Theinteger and scale attributes discussed in “Data attributes” on page 321 can helpyou do this.Additional operations on decimal data are provided by several of the instructionsin “General Instructions” in the applicable Principles of Operation manual. Decimaloperands always reside in storage.For further information, see “Decimal Instructions” in the applicable Principles ofOperation manual.Floating-pointinstructionsUse floating-point instructions when you want to do arithmetic operations on datain the floating-point representation. Thus, you do not have to keep track of thedecimal point in your computations. Floating-point instructions also let you doarithmetic operations on both very large numbers and very small numbers, usuallyproviding greater precision than fixed-point decimal instructions.For further information, see “Floating-Point Instructions” in the applicablePrinciples of Operation manual.ControlinstructionsControl instructions include all privileged and semiprivileged machine instructions,except the input/output instructions described on page 76.Privileged instructions may be processed only when the processor is in thesupervisor state. An attempt to process an installed privileged instruction in theproblem state generates a privileged-operation exception.Semiprivileged instructions are those instructions that can be processed in theproblem state when certain authority requirements are met. An attempt to processan installed semiprivileged instruction in the problem state when the authorityrequirements are not met generates a privileged-operation exception or some otherprogram-interruption condition depending on the particular requirement that isviolated.For further details, see “Control Instructions” in the applicable Principles ofOperation manual.Input/outputoperationsUse the input/output instructions (instead of the IBM-supplied system macroinstructions) when you want to control your input and output operations moreclosely.76 HLASM: V1R6 Language Ref