Registers on entry

Standard OS linkage conventions are used, and the registers on entry to the filter module are:

R13 contains the address of a standard 18-word save area
R14 contains the return address to the exit
R15 contains the filter's entry point address
R1 contains the address of a list of six fullwords that address:
1. A fullword containing the exit_type
2. A fullword integer containing the action code
3. A fullword integer where the filter puts the return_code
4. A 4-fullword handle area
5. A fullword integer containing the ADATA record_length
6. The ADATA record
  The high-order bit of the last fullword address is set to one.
Figure 1 shows the six fullwords in the parameter list.

Figure 1. Filter module parameter list format

                                                              ┌───────────────┐
                                                     ┌───────>│ exit_type     │
                                                     │        └───────────────┘
                                                     │
                                                     │        ┌───────────────┐
                                                     │ ┌─────>│ action        │
 ┌───────────────┐      ┌──────────────────────────┐ │ │      └───────────────┘
 │ register 1    ├─────>│ address of exit_type     ├─┘ │
 └───────────────┘      ├──────────────────────────┤   │      ┌───────────────┐
                        │ address of action        ├───┘ ┌───>│ return_code   │
                        ├──────────────────────────┤     │    └───────────────┘
                        │ address of return_code   ├─────┘
                        ├──────────────────────────┤          ┌───────────────┐
                        │ address of handle        ├─────────>│ handle        │
                        ├──────────────────────────┤          └───────────────┘
                        │ address of record_length ├─────┐
                        ├──────────────────────────┤     │    ┌───────────────┐
                        │ address of record        ├─┐   └───>│ record_length │
                        └──────────────────────────┘ │        └───────────────┘
                                                     │
                                                     │        ┌───────────────┐
                                                     └───────>│ record        │
                                                              └───────────────┘