SWI Error Block
Programmer's Reference Manuals
» Introduction To SWIs
» SWI Error Block
SWI error blocks are the key mechanism by which errors are returned from SWIs. Whenever a SWI call exits with the V flag set, R0 will contain a pointer to an error block, with the following structure:
| Offset | Contents |
|---|---|
| +0 | Error number |
| +4 | Error String (zero terminated) |
The error number is broken down as follows:
| Bits | Meaning when set |
|---|---|
| 0-7 | Basic error number |
| 8-23 | Error owner or Error Generator |
| 24-29 | Reserved. Must be 0 |
| 30 | Defined to be 0 |
| 31 | Error is serious (hardware). Owner is then; |
| 0 Machine failure (data abort, undefined instruction, etc.) | |
| 1 Co-Processor failure | |
| 2 Floating Point (FPA) | |
| 3 Econet | |
| 6 Floating Point (VFP) |
Notes
X SWI’s are allowed to generate (rather than return) serious errors.
Starting with RISC OS 5.23, whenever a SWI exits with the V flag set, some sanity checks will be performed on R0 to try and make sure it’s a valid error block pointer. This includes sanity-checking the pointer value, as well as the error number (e.g. making sure no reserved bits are set). These checks are designed to help catch programming mistakes before they are able to cause problems elsewhere – e.g. on a system running with high processor vectors, a null error pointer will result in a crash. Without help from the kernel, the SWI which generated the bad error pointer may be hard to track down.
If a bad error pointer is detected then it will be swapped for a pointer to a different error block, with the error message describing which SWI is at fault.
For compatibility with existing programs, XOS_GenerateError is exempt from SWI error pointer validity checks (it is essentially a no-op as far as error generation is concerned, and many programs take advantage of this and abuse the SWI by using it for other purposes, e.g. as a method of triggering callbacks or converting null-terminated strings to BASIC form). Similarly, XMessageTrans_ErrorLookup is exempt from error number validity checks, as some code operates by passing in a dummy error number and then replacing it with a valid one in the returned error block (although often this isn’t necessary and the correct error number could have just as easily be placed in the input block). All other SWIs, whether called in X or non-X forms, will have the error pointer check performed.
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