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RACF (Resource Access Control Facility)

RACF can be though of as the database that contains all of the securuity policies for the system. z/OS uses security policies in RACF to decide if someone requesting access to a resource should have access or not.

  • This is a security manager for z/OS that maintains security policies that define who and what kind of access users have to resources on the system.


  • This is a z/OS macro that is used to interface with SAF. In other words, this macro is the interface for making security related requests to the system.

RAS (Reliability, Availability, Serviceability)

  • These are guidelines and standards for ensuring that applications are production ready.

Real Storage

  • Real physical view of storage/memory accessible to an LPAR. There is no abstraction or logical view in this context. In other words, real storage is storage/memory as the z/OS operating system running on an LPAR sees it.

RECFM (Record Format)

Think of punch cards. A punch card represents one line or stream of bytes that contains 80 characters/bytes. So, if you were to read a series of punch cards into a data set, you would define the record format of that data set as fixed (F) or fixed blocked (FB) since each punch card has a fixed length of 80 characters/bytes per record/line.

  • This is a data set attribute that defines how data is organized in a data set.


  • Data sets are composed of a series lines or records. This is different from Unix files which are just byte streams that use the \n character to indicate the start of a new line. For data sets, however, lines/records are an architected data structure. When a user reads or writes to a data set, the operating system treats and interprets all of the lines as individual lines/records that make up the entire data set instead of a contiguous byte streams like on Unix based systems.

RELFILE (Relative File)

Think of the metadata files that come with a Python or Debian package.

  • These are the files that describe the layout of a product so that SMP/E knows how to find and install the files contained in the product.


  • In SMP/E, REQ is specified as part of the ++VER SMPMCS statement to define an FMID/product that the FMID/product being installed requires.

Rexx (Restructured Extended Executor)

Rexx is a lot like Python.

  • High level programming language developed at IBM UK.

RMF (Resource Measurement Facility)

Think of Task Manager on Windows.

  • Optional z/OS product used for measuring system performance.

RMODE (Residency Mode)

Think 32 bit and 64 bit intel.

  • Defines what address ranges a program can run in (i.e. 24 bit, 31 bit 64 bit). For example, a RMODE24 program cannot run in an area in memroy/storage that is not addressable with 24 bits.

RRDS (Relative Record Data Set)

This is functionally similar to an array in high level programming languages where elements are accessed by their index.

  • A VSAM data set where data is accessed by it’s record number where each record number is relative to the first record.