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

DE Contains a pointer to the File Control Block used to open the program file
being run. This may prove useful to access the program file as data since
the file is already in an open condition (the PRO-PaDS utility from MISOSYS
makes use of this condition).
HL Contains a pointer to the first non-blank character on the command line
which terminated the parsing of the name entered in order to execute the
program. This pointer should be used if you are going to parse command-line
file specifications using @FSPEC or parameters via @PARAM.
If the program was executed from DOS Ready or via @CMNDI, the stack pointer (SP) will
point to the system stack which has approximately 150 bytes of storage space. If the
program was executed via @RUN or @CMNDR, then the stack pointer contains whatever was
established by the invoking program. In any event, the top of the stack will contain the
return address to the module which is invoking the program, be it another program or the
system.
If you are going to switch stacks, you should be aware that the system's task processor
requires possibly 40 bytes of stack space. The exact amount will depend on what tasks are
active. Release 6.0.0 of the DOS also has a restriction that limits the stack to reside
below X'F400'. If you are going to use the @BANK request to toggle memory banks, then the
stack must reside below X'7FFE'.
When your program terminates, it should load register pair HL with a return code. If the
program terminates without error, use a return code of 0. If the termination is due to a
DOS I/O error or other error being returned by an SVC as noted in the error dictionary,
load that error number into HL. For all other error conditions, the suggested procedure
is to load a -1 (X'FFFF') into register pair HL. After loading HL, you can either issue a
RET instruction or issue an @EXIT SuperVisor Call. Note that the RET exit method mandates
that you maintain the integrity of the stack pointer so that it is pointing to a valid
return address. You may want to establish exit code that reloads the stack pointer with
the SP contents that you saved when first executing the program. Thus, the SP will always
be correct for an RET. An @EXIT termination will always restore control to the operating
system even if the program was invoked via an @CMNDR. Therefore, if you suspect that your
program will be invokable from another program, you should use the RET method for program
termination.
7.3 SUPERVISOR CALLS LISTED ALPHABETICALLY
Name Svc # Purpose
@ABORT SVC-21 Abnormal program exit
@ADTSK SVC-29 Add a task process
@BANK SVC-102 RAM bank switching
@BKSP SVC-61 File record backspace
@BREAK SVC-103 Establish vector
@CHNIO SVC-20 Device chain character I/O
@CKBRKC SVC-106 Check for a keyboard BREAK
@CKDRV SVC-33 Check disk drive availability (& log)
@CKEOF SVC-62 Check for file's end-of-file (EOF)
@CKTSK SVC-28 Check task slot availability
@CLOSE SVC-60 Close an open disk file
@CLS SVC-105 Clear the Video screen
@CMNDI SVC-24 Interpret and execute a command
@CMNDR SVC-25 Execute a command and return
@CTL SVC-05 Control a device chain
@DATE SVC-18 Obtain system date
@DCINIT SVC-42 Initialize a disk controller
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