Error Handling; Program Execution Of Commands - ZiLOG Z80-RIO Operating System User's Manual

ing from 0 to the beginning of the loaded file. If both attempts
fail, no stack space is available and command execution cannot
be initialized. Thus, normally the user stack is located im-
mediately after the loaded file.
Since it is more efficient to not repeatedly load a command
file which is to be executed several times in succession,
RIO remembers the entry point of the last loaded file and
provides the internal command XEQ (see ·section 5.42) to
transfer program execution to that addres~. Most RIO
commands can be executed repeatedly in this way.
Prior to executing the external command file, the memory
map is examined to identify those segments which were
allocated as a result of loading the file(s) to be
executed. When return to OS is made, these segments will
be deallocated. Note that in the event a file is loaded,
but not executed, the space it occupies will be allocated
until either explicitly deallocated (see the RELEASE
command, section 5.35) or a return to OS is made from any
external command file.
2.5 ERROR HANDLING
Wherever errors occur in the processing or execution of
system commands, a message is directed to the console
output device. Command processing then continues with the
next command in the command string, if any.
2.6 PROGRAM EXECUTION OF COMMANDS
Any command or user program executable from the system
console can also be executed from a program. This is
accomplished by making a system call to RIO with reference
to the command string to be executed. In this way programs
can be chained together or complex overlay structures
easily implemented. (See the Relocating Assembler and
Linker manual for details of overlay creation.) System
calls and the RIO system entry point are described in
sections 3.2 and 4.5.
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