|Advanced Data Set Information||TSO Support|
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.
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.
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.
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.
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
The readme file is in folder \mfuser\projects\demo\ctlblkc.
The source files are in folder \mfuser\projects\demo\ctlblkc\source.
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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|Advanced Data Set Information||TSO Support|