F$Chain

Chain	Loads and executes a new primary module without creating a new process
OS9 F$Chain 103F 05

Entry Conditions
A	language/type code
B	size of the area (in pages); must be at ???
X	address of the module name or filename
Y	parameter area size (in pages); defaults to zero if not specified
U	starting address of the parameter area; must be at least one page
Error Output
CC	carry set on error
B	error code (if any)

Additional Information

• Chain loads and executes a new primary module, but does not create a new process. A Chain system call is similar to a Fork followed by an Exit, but it has less processing overhead. Chain resets the calling process program and data memory areas and begins executing a new primary module. It does not affect open paths. This is a user mode system call.
• Warning: Make sure that the hardware stack pointer (Register SP) is located in the direct page before Chain executes. Otherwise the system might crash or return a suicide attempt error. This precaution also prevents a suicide in the event that the new module requires a smaller data area than that in use. Allow approximately 200 bytes of stack space for execution of the Chain system call.
• Chain performs the following steps:
  1. It causes NitOS-9 to unlink the process’s old primary module.
  2. NitOS-9 parses the name string of the new process’s primary module (the program that is to be executed first). Then, it causes NitrOS-9 to search the system module directory to see if a module with the same name, type, and language is already in memory.
  3. If the module is in memory, it links to it. If the module is not in memory, it uses the name string as the pathlist of a file to load into memory. Then, it links to the first module in this file. (Several modules can be loaded from a single file.)
  4. It reconfigures the data memory area to the size specified in the new primary module’s header.
  5. It intercepts and erases any pending signals.

The following diagram shows how Chain sets up the data memory area and registers for the new module.

+----------------+ <-- Y (highest address)
| Parameter Area |
|----------------| <-- X, SP
|                |
|                |
|    Data Area   |
|                |
|                |
|----------------|
|                |
|   Direct Page  |
|                |
+----------------+ <-- U,DP (lowest address)

D	parameter area size
PC	module entry point absolute address
CC	F=0, I=0; others are undefined

Registers Y and U (the top-of-memory and bottom-of-memory pointers, respectively) always have values at page boundaries. If the parent process does not specify a size for the parameter area, the size (Register D) defaults to zero. The data area must be at least one page.

(For more information, see the Fork system call.)

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