The Request Code interface operates in a manner similar to the binder API in that a series of calls is required to extract data; at a minimum the application must make one call to start a session, one to get data, and one to end the session. The caller provides a parameter list for each call that specifies the service being requested. This is a standard call using OS linkages with two qualifications:

• The parameter lists must be constructed in the variable length form, with the high order bit set on the last parameter. In assembler this would be done using LINK or (preferably) CALL with the VL parameter. High level languages typically, though not universally, follow this convention by default.
• The program being called, IEWBFDAT, must not be resolved statically when the application is created, and is not a DLL. It is located in the MVS™ Link Pack Area and needs to be located dynamically during execution. In assembler language this is done using LOAD or LINK. High Level Language techniques for accomplishing this vary.

Every call provides at least three pieces of information:

• A function code. Two upper case EBCIC alphabetic characters.
• An interface level. Currently always 1.
• An mtoken. See Mtoken.

Upon return from fast data access, you can examine the return and reason codes. Fullword return and reason codes are returned in registers 15 and 0 respectively. For languages that do not provide access to one or both of those registers on return, an RC service call is provided, which returns the most recent return and reason codes in fields passed as parameters to the RC service call.