User`s guide

1.3.3 Applications and RSXs

A Resident System Extension is a temporary additional operating system module.

An RSX can extend or modify one or more operating system functions selectively.

As with a standard BDOS function, a transient program accesses an RSX function

through a numbered function call.

At any one time there might be zero, one, or multiple RSXs active in memory.

When a transient program makes a BDOS function call, and RSXs are active, each

RSX examines the function number of the call. If the function number matches the

function the RSX is designed to extend or modify, the RSX performs the requested

function. Otherwise, the RSX passes the function request to the next RSX. Noninter-

ceptcd functions are eventually passed to the BDOS for standard execution.

RSXs are loaded into memory when programs containing RSXs are loaded. The

CP/M 3 utility, GENCOM, can attach RSXs to program files. When attaching RSXS,

GENCOM places a special one page header at the beginning of the program file. The

CCP reads this header, learns that a program has attached RSXS, and loads the RSXs

accordingly. The header itself is not loaded into memory; it merely indicates to the

CCP that RSX loading is required.

The LOADER module is a special type of RSX that supports BDOS function 59,

Load Overlay. It is always resident when RSXs are active. To indicate RSX support

is required, a program that calls function 59 must have an RSX header attached by

GENCOM, even if the program does not require other RSXS. When the CCP

encounters this type of header in a program file when no RSXs are active, it sets the

address at location 0006H in Page Zero to LOADER-base + 6 instead of

BDOS_base + 6.

1.4 Memory Region Boundaries

This section reviews memory regions under CP/M 3, and then describes some

details of region boundaries. It then relates the sizes of various modules to the space

avialable for the execution of transient programs. Figure 1-6 reviews the location of

regions in logical memory.

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1.3 Component Interaction CP/M 3 Programmer's Guide