CD Shell Development: Function Services

Function Services

Function Services List

Modules are able to present a list of function services to the core. The list is a linear array made up of individual function service definitions, each of which define a single function supported by the module. These functions are then used to evaluate parameter strings into numeric results, for use in mathematical expressions.

The definitions in the function services list must be in alphabetical order by function name. A 2-byte (16-bit) zero value terminates the list.

Function Service Definition

Offset	Size	Description
0	2 bytes	service routine pointer
2	2 bytes	flags
4	2 bytes	name pointer
6	2 bytes	description text pointer

Service Routine Pointer

This is the offset into the module segment of the handler routine that processes this function.

Flags

This field is a set of flags that determine how the function is handled. The bitfields are defined as follows:

Bit	Field	Description
[0]	Ignore	pretend this function is not in the list
[1]	Hide	don't display info about this function
[15:2]	Reserved	these flags are reserved for future use

Name Pointer

This is the offset into the module segment of the function name, a null-terminated string.

Description Text Pointer

This is the offset into the module segment of the function description, a short string presented to the user in the master function list. If no description is provided, this field should be zeroed out.

Service Routine Environment

Call Context

The function service routine is called by the core with the following context:

Register	Contents
si	offset of parameter string (treat string as read-only)
cs	module segment
ds	segment of parameter string (core segment)
es	module segment
gs	Core Services Table segment
ss:sp	global stack, far return pointer on top

Return Context

The core expects the following context upon return from the service routine:

Register/Flag	Contents
carry flag	set if function evaluation failed
eax	value of function
ss:sp	global stack, far return pointer popped

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