The Programmer's Guide to TRSDOS Version 6 - Tim Mann's Home ...

8.11 SYSTEM OVERLAY CONTENTS
A system as complex and flexible as LDOS would occupy considerable memory space to be
able to provide all of its features. The DOS, however, makes extensive use of overlay
segments in order to minimize the amount of memory reserved for system use. The
compromize in using an overlay driven system, is that while a user's application is in
progress, certain disk file activities requested of the system may require the operating
system to load different overlays to satisfy the request. This could cause the system to
run slightly slower than a less sophisticated system which has more of its file access
routines always resident in memory. The system provides a procedure to permanently place
specified overlays into memory to enhance the overall speed of operation (see the SYSTEM
command).
The following will describe the functions performed by each system overlay. Numbers in
angle brackets represent the system SVC entry.
8.11.1 SYS0/SYS
This is not an overlay. It contains the resident part of the operating system (SYSRES ).
8.11.2 SYS1/SYS
This overlay contains the command interpreter. This processes @CMNDI and @CMNDR
. It contains the routines for processing the @FEXT SVC , the routines for
processing the @FSPEC SVC , and the routines for processing the @PARAM SVC
. It also contains the @EXIT processor .
8.11.3 SYS2/SYS
This overlay is used for opening or initializing disk files and logical
devices. It contains the functions for @RENAM and @GTDCB . It also
contains the @CKDRV routines , and routines for hashing file specifications
and passwords .
8.11.4 SYS3/SYS
This overlay contains all of the system routines needed to close files and devices
. It also contains the routines needed to service the @FNAME SVC .
8.11.5 SYS4/SYS
This system overlay contains the system error dictionary and @ERROR SVC processing
routines.
8.11.6 SYS5/SYS
This overlay contains the system debugger.
8.11.7 SYS6/SYS
This overlay contains all of the algorithms and routines necessary to service the LIBrary
commands identified as "Library A" by the LIB command. The following list identifies the
commands and their ISAM entry number.
21 DIR 63 RESET 53 RENAME
61 DEVICE 65 SET 1E MEMORY
32 COPY 66 FILTER 91 DO
31 APPEND 41 LIST 81 LOAD
64 ROUTE 18 REMOVE 82 RUN
62 LINK 19 LIB
8.11.8 SYS7/SYS
This overlay contains all of the algorithms and routines necessary to service the LIBrary
commands identified as "Library B" by the LIB command. The following list identifies the
commands and their ISAM entry number.
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