System information

Transient programs are loaded into the TPA and executed as follows. The operator
communicates with the CCP by typing command lines following each prompt. Each command
line takes one of the following forms:
command
command file1
command file1 file2
where command is either a built-in function, such as DIR or TYPE, or the name of a transient
command or program. If the command is a built-in function of CP/M, it is executed immediately.
Otherwise, the CCP searches the currently addressed disk for a file by the name
command.COM
If the file is found, it is assumed to be a memory image of a program that executes in the TPA
and thus implicity originates at TBASE in memory. The CCP loads the COM file from the disk
into memory starting at TBASE and can extend up to CBASE.
If the command is followed by one or two file specifications, the CCP prepares one or two File
Control Block (FCB) names in the system parameter area. These optional FCBs are in the form
necessary to access files through the FDOS and are described in Section 5.2.
The transient program receives control from the CCP and begins execution, using the I/O
facilities of the FDOS. The transient program is called from the CCP. Thus, it can simply return
to the CCP upon completion of its processing, or can Jump to BOOT to pass control back to
CP/M. In the first case, the transient program must not use memory above CBASE, while in the
latter case, memory up through FBASE-1 can be used.
The transient program can use the CP/M I/O facilities to communicate with the operator's
console and peripheral devices, including the disk subsystem. The I/O system is accessed by
passing a function number and an information address to CP/M through the FDOS entry point at
BOOT+0005H. In the case of a disk read, for example, the transient program sends the number
corresponding to a disk read, along with the address of an FCB to the CP/M FDOS. The FDOS,
in turn, performs the operation and returns with either a disk read completion indication or an
error number indicating that the disk read was unsuccessful.
5.1 Introduction CP/M Operating System Manual
5-3