System Control Task
The system control task consists of
nonexecutable modules, as follows:
five executable and two DMTREX Handle console I/O; process request elements for service routines; terminate system service and line driver tasks.
DMTCRE DMTCMX DMTMGX DMTCOM DMTMSG DMTSYS Start a line driver task and create the rMTAXS and DMTLAX tasks during initialization.
Handle all console functions.
Build and forward message request elements. Perform miscellaneous system service functions.
Table of message texts and codes.
Link table, file tag storage area, tag queue pointers,
and switched line port table.
Free Storage and Line Drivers
This area of free storage is also managed by DMTSTO. In addition to
providing storage for system tasks, it is used for line driver storage.
For each active link that is initialized by DMTCRE, a copy of a DMTSML or DMTNPT line driver is brought into virtual storage. Line driver
storage is assigned downward from X'7COOO', in four-page increments.
Free storage for system tasks is assigned upwards from the page boundary
following DMTSYS, in one-page increments.
Line Allocation Task
The DMTLAX module allocates a line port to a link when its line driver
task is started. If a line address has been previously assigned in the
link definition or is specified in the START command, DMTLAX verifies
that the line is for a valid device type and is not already in use. If
a line address has not been previously assigned and is not specified in
the START command, DMTLAX scans the table of switchable line ports fer
an available line and assigns it to the link's line driver task. If a
line is not available or is incorrectly specified, an error message is
issued to the RSCS operator. Spool File Access Task
The DMTAXS module accepts files from the VM/370 spool system and
maintains the queues of main storage file tag slots; executes the ORDER, CHANGE, and PURGE commands; and opens and closes input and output VM/370 spool files.
334 IBM VM/370 System programmer's Guide

Functional Information
The RSCS virtual machine performs certain basic functions as it manages
the transmission of files between the host VM/370 and remote locations.
These functions include: • Virtual storage management • File management • Task-to-task communication • RSCS command processing • RSCS message handling • Interrupticn handling
Virtual Storage Management
The RSCS supervisor controls virtual storage in blocks of either 4096 bytes (page size) or in 16-byte queue elements. Tasks running under the
supervisor obtain their working storage area in page size blocks and
then allocate variable size blocks as their functions require. PAGE ALLOCATION Page allocation is performed by the supervisor service routine, DMTSTC. A storage allocation map, 256 bytes in length, is located in the
supervisor area and is pointed to by MAINMAP in the DMTVEC data area.
Each byte represents a page of virtual storage and contains X'OO' if the
page is free. MAINSIZE, also in DMTVEC, contains the total number of
pages defined for the particular RSCS virtual machine. When a task requires a page of storage, it first searches the storage
allocation maF for a free page (X' 00') • The page number is plac,ed in register 1 and a call to DMTSTO reserves the page. replaces the
storage map byte with the one-byte TASKID assigned to the calling task
by the supervisor. To release storage, a task has only to clear the
appropriate bytes in the storage map. QUEUE ELEMENT MANAGEMENT with the exception of a few words of low address storage by the
dispatcher, the rest of the supervisor status information is stored in
chains of 16-byte queue elements managed by DMTQRQ. The first extent of
these queues is in the supervisor and occupies the area between the main
storage allocation map and DMTEXT. A supervisor queue extension area,
one page in length, lS located de X!1COO!. Queue elements are dequeued
from the free element queue pointed to by FREEQ in DMTVEC and enqueued
on one of the active queues (TASKQ, MPXIOQ, SELIOQ, IOEXTQ, EXTe, ALERTQ, or GIVEQ). When the queue element is released, it is returned
to the free element queue.
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