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

LD (IX+2),15
JR TASKA
By firmly understanding the functions of each of the TP SuperVisor Calls discussed, you
will be proficient at integrating interrupt tasks into your applications. A final note is
to be aware of the task slots already used by the DOS or other applications. Use @CKTSK
to find an unused task slot.
8.7 LOW MEMORY DETAILS
The author thought long and hard concerning the inclusion of this section of the
Appendix. Why is this section a problem? The Version 6 operating system was designed to
promote the development of portable software. The term, portable, means not only should
the software function from machine to machine, it should also function under each release
of the DOS. The DOS needs access to the storage of data for internal system use. Trying
to keep the memory locations of this data constant across all implementations of the
system is quite restrictive and usually becomes limiting to the healthy growth of the
system. Keeping portability in mind, the designers of the system have provided SuperVisor
Calls which return pointers to data that may be useful to a program. Thus, there should
usually be no need to access data areas by memory address. We say "usually" since it is
possible that user's of the system are writing machine-dependent SYSTEM code. This is the
only reason that the Appendix contains this section. It is recognized that once a data
address is known, application programmers tend to use it. RESIST THE IMPULSE. If the
system does not provide via an SVC, data that you think you need, perhaps you don't
really need the data. It is entirely possible that the information you need is actually
available via an SVC, although not entirely obvious. Remember, when you bypass the SVC
structure of the DOS, you most certainly risk portability!
With the preceding discussion in mind, let's first take a look at the general uses of
each low core memory page.
Sector Page General Contents
n/a 0 RST vectors, Flag tables, misc...
n/a 1 SuperVisor Call Table
0 2 Bank data, 31 Device Control Blocks
1 3 System stack area, Miscellaneous machine dependent routines.
2 4 System Information data, Drive Control Table, Input buffer.
3 5 Start of I/O handling and drivers. Extends to end of page
12H.
The low core area starting at memory page two is actually loaded by and from the
BOOT/SYS. The system uses the first two sectors to contain BOOT code needed to bring up
the system. A booting ROM reads either the first or second sector of track 0 - the BOOT
track. This sector contains code which, in turn, reads the entire BOOT/SYS file.
Thereafter, BOOT loads the resident system file, SYS0/SYS, and transfers control to it.
Because of this process, part of low memory is loaded directly from the BOOT/SYS file
contained on track 0 while other parts of low memory are loaded by SYS0/SYS. A
description of the booting process and the boot track is contained in another section of
the Appendix. Let's now look at some of the details of low memory. REMEMBER THAT THIS
INFORMATION IS PROVIDED FOR USE ONLY IN EXTREME NON-PORTABLE SITUATIONS!
An asterisk following the page byte(s) indicates a quantity that can be obtained from the
system via some SuperVisor Call. A pound sign indicates that the address is fixed due to
processor assignment.
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