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

NAME START END DESCRIPTION
LOWCORE X’0000 @$SYS RST vectors, NMI vector, System flags, Date, Time,
System FCB, DEBUG register save area, JCL FCB,
Command FCB, SVC Table, DCB Table, System stack,
Miscellaneous data, Command input buffer, Drive
Control Table, Device I/O handler, Clock task, Memory
management routines.
IOR @SYS X’12FF Keyboard, Video, Printer, and Disk drivers.
SYSRES X’1300 X’1DFF File access routines, SVC processor, System overlay
handler, System program loader, Interrupt Task
Schedular, System buffer.
SOR X’1E00 X’23FF Execution region for system overlays 2-5, 9-13,
overlay disk file buffer.
LOR X’2400 X’25FF Execution region for system library comands contained
in libraries A, B, & C.
UPR X’3000 (HIGH$) Execution region for user transient programs (note:
programs not accessing the system libraries can start
at X'2600'.)
HIMEM (HIGH$)+1 X’FFFF Region for relocation of extended system and user
static modules.
Table 2-1 System Map
The task scheduler is also used by the despooling function of the printer spooler. The
DOS spooler implements a combination of memory and disk buffers to temporarily hold the
printer output. This output is despooled to the printer under the control of the task
scheduler. The function, being transparent to the user, can continue the despooling even
after the application generating the output is finished and another started. When the
system contains 128K (or more) of RAM, the extra RAM can be set aside for the spooler's
memory buffer.
The primary function of any operating system is to provide the user with a facility for
managing and accessing files stored on disk storage devices. Since the user must not be
burdened with the physical details of the storage devices themselves, it is the operating
system's responsibility to translate all file record access requests into specific drive,
track, sector, and head parameters that pinpoint the storage location of each record. The
DOS supports a wide range of disk storage capacities. Let's take a brief look at how a
disk drive is organized.
Each track is formatted into a specific quantity of 256-byte sectors with a maximum
capacity of 32 sectors per track. Sectors are grouped into blocks called "granules" which
vary in size according to total track capacity. Whenever additional disk space is needed
for a file, an additional granule is allocated. The granule thus becomes the minimum size
storage unit. Where multiple headed drives are in use, the track numbers on a surface are
duplicated on each surface with all similarly numbered tracks constituting a cylinder.
Cylinder capacities also have an upper limit of 256 sectors per cylinder or eight
granules per cylinder while the system supports a maximum of eight heads per drive.
In order to evenly use the entire surface of a drive, files are uniformly distributed
across each surface [note: LSI unfortunately has changed to a fixed allocation scheme
effective with release 6.1]. That means the head has a tendency to be randomly located
whenever a directory access is needed. Because of this, each disk drive's directory is
placed on the cylinder closest to its midpoint which provides a tendency to minimize the
average seek time for directory accesses. The directory, of course, contains information
on each file stored on the drive as well as additional tables and codes pertinent to the
drive.
The first sector of the directory contains a granule allocation table (GAT). The GAT is
bit mapped to each granule of space on the drive. Other fields in the GAT contain the
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