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Chapter 5: MVS Control Blocks

Mainframe Express emulates a number of MVS control blocks that are accessible to applications running in problem program state. The information stored in these control blocks is limited to that which has value and meaning to problem state programs. This chapter describes how these control blocks operate differently in a PC environment compared to a mainframe.

5.1 Access to the Control Blocks

Control blocks are accessed in Mainframe Express in the same way as they are on a mainframe, by referencing the appropriate field in the Communication Vector Table (CVT). The address of the CVT is stored at absolute address 16 in memory, as it is on the mainframe.

5.1.1 Control Block Structure

MVS control blocks in Mainframe Express have the same structure as the mainframe versions, but only those fields most likely to be accessed by Assembler application programs are supported.

5.1.2 Low-memory Address Emulation in Assembler

Many absolute memory addresses are used by the MVS operating system and the mainframe hardware itself. The Assembler run-time system under Mainframe Express intercepts all accesses to memory locations 0 through 2047 and redirects them to a block of memory that is allocated from the PC operating system and initialized to emulate the bottom 2,048 bytes of memory on the mainframe. Therefore, no matter where in PC memory an Assembler program is loaded, absolute addresses 0 through 2047 are guaranteed to exist and can be accessed and modified as on the mainframe.

5.1.3 Sample Project

The following sample project is supplied with Mainframe Express to illustrate how to access the MVS control blocks. A readme.txt file is supplied which describes how to create the project and run it.

CTLBLKC The CTLBLKC project contains a COBOL program which calls an Assembler routine to extract information from the MVS control blocks.

The readme file is in folder \mfuser\projects\demo\ctlblkc.

The source files are in folder \mfuser\projects\demo\ctlblkc\source.

5.2 Control Blocks Supported

The following table lists the MVS control blocks that are emulated by Mainframe Express and specifies any differences between the PC versions and the mainframe versions.

Mainframe Express supplies macros for Assembler to define the MVS control blocks. They are located in the folder mfe\source. The macros have the same name as the control blocks and have the .mac file extension. They describe the key problem program state fields. Only fields that are supported are named in the macros; the others are defined as "fillers".

Name Description Differences from mainframe
IHAACEE Accessor Control Element ACEEVRSN is set to 2
IHAASCB Address Space Control Block  
IHAASVT Address Space Vector Table  
IHAASXB Address Space Extension Block  
CVT Communication Vector Table

CVTTOD is set to X'20', indicating that the Time Of Day clock is operational.

CVTMVSE - the X'80' bit in CVTDCB is always on, indicating that you are running under MVS/XA or later.

CVT4MPS - the X'04' bit in CVTDCB is on (indicating that you are running on System/360 Model 65 MP) only if the MVS control blocks are above the 16MB line (that is, all three-byte addresses are stored as four-byte addresses in the same location).

CVTDATE is set with the century number (that is, it uses "0cyyddds" format, where "c" is century number - 0 for 20th, 1 for 21st, etc.)

CVT CVT Prefix The CVT prefix is set to all binary zeros.
IHACDE Contents Directory Entry One "null" CDE is allocated. So it appears to be using a program for which the load module has not been loaded.
IDADDSA Dynamic Storage Area (FSA - First Save Area) This is an 88-byte save area pointed to by TCBFSA. It is used as the TCB First Save Area.
IEFJFCBN Job File Control Block  
IEZJSCB Job Step Control Block  
IHAPSA Prefixed Save Area The X'01' bit in byte 7 (the first byte being byte 0) is on only if the MVS control blocks are above the 16MB line (that is, all three-byte addresses are stored as four-byte addresses in the same location).
IEESMCA SMF Control Area SMCASID is set to 'MSTR'.
IDADTCA Task Communication Area

The PRV address is always zero.

The storage overflow routine does not provide overflow segments; it merely abends with the abend code S80A. You must ensure that the ISA setting is adequate for your application.

IKJTCB Task Control Block The TCB protect key is set to key 8.
IEFTIOT1 Task I/O Table  

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