VFPSupport
VFPSupport API – for VFPSupport 0.02
VFPSupport_CheckContext
(SWI &58EC0)
| Entry | |
|---|---|
| R0 | Flags |
| b0 = User mode flag (0=user mode access not required, 1=user mode access required) | |
| b1 = Application space flag (0=context not in application space, 1=context in application space) | |
| b30-31 ignores (for CreateContext compatability) | |
| Other bits reserved, sbz | |
| R1 | Number of doubleword registers required (1-32) |
| Exit | |
|---|---|
| R0 | Required size of context save area |
| - | Other registers preserved |
Checks if it’s possible to create a context with the indicated settings, and how much memory is required for the context save area if the application is to manage memory allocation itself. The size of the context save area (for a particular combination of R0 and R1) will not change at runtime, but is subject to change across different machines or OS versions.
Returns an error if the required features are unavailable.
VFPSupport_CreateContext
(SWI &58EC1)
| Entry | |
|---|---|
| R0 | Flags |
| b0 = User mode flag (0=user mode access not required, 1=user mode access required) | |
| b1 = Application space flag (0=context not in application space, 1=context in application space) | |
| b30 = Activate lazily flag (0=don’t activate lazily, 1=activate lazily) | |
| b31 = Activate flag (0=leave context inactive, 1=activate now) | |
| Other bits reserved, sbz | |
| R1 | Number of doubleword registers required (1-32) |
| R2 | Pointer to word-aligned context save area of the size indicated by VFPSupport_CheckContext, or 0 if VFPSupport is to allocate memory itself |
| R3 | FPSCR value to initialise context with |
| Exit | |
|---|---|
| R0 | Context pointer |
| R1 | Previously active context (iff R0 b30 or b31 == 1 on entry, else preserved) |
| - | Other registers preserved |
Creates and potentially activates a context.
If R2 is supplied, the parameters in R0 and R1 must be identical to those passed to VFPSupport_CheckContext when determining the context size (with the exception of the high-bit flags which CheckContext is documented as ignoring). Use bit 1 of R0 to indicate whether the context is in application space or not (or whether it’s in some other area of memory that will be paged in/out by the Wimp). If the context isn’t in a paged area that’s managed by the Wimp, then it must be in a non-paged area. If R2 is supplied then the context pointer returned in R0 will be identical to the value of R2 given on entry.
If R2 isn’t supplied then VFPSupport will allocate the required amount of memory for you. In this case, bit 1 of R1 must be clear.
Bits 30 and 31 of R0 control whether the context should be immediately activated or left inactive. If bit 30 is set then bit 31 is ignored.
Returns an error if the required features are unavailable.
Note that if R2 is supplied then fewer error checks than normal may be performed, under the assumption that the caller correctly responded to any errors returned by CheckContext.
VFPSupport_DestroyContext
(SWI &58EC2)
| Entry | |
|---|---|
| R0 | Context ID |
| R1 | Context to activate if R0 was the active context |
| Exit | |
|---|---|
| R0 | Context that’s now active (whether R0 was active or not) |
| - | Other registers preserved |
Destroys a context, potentially activating the indicated context if the context being destroyed was the active one.
Returns error if context not found.
If an error is returned, the active context will not be modified.
VFPSupport_ChangeContext
(SWI &58EC3)
| Entry | |
|---|---|
| R0 | Context to activate |
| R1 | Flags |
| b0 = Lazy activation (0=activate now, 1=use lazy activation) | |
| b1 = Application space changing flag (0=no change, 1=changing) | |
| other bits reserved, sbz |
| Exit | |
|---|---|
| R0 | Previously active context |
| - | Other registers preserved |
Activates the specified context, potentially using lazy activation.
If bit 1 of R1 is set then it indicates that application space is about to be changed. If this bit is set, and any active context is flagged as being in application space, then a nonlazy context switch will occur, regardless of the setting of R1 bit 0. This allows the Wimp to safely use lazy context switching when deactivating contexts during task switches.
Returns error if context not found.
If an error is returned, the active context will not be modified.
VFPSupport_ExamineContext
(SWI &58EC4)
| Entry | |
|---|---|
| R0 | Context |
| R1 | Flags |
| b0 = Serialise context | |
| Other bits reserved, sbz |
| Exit | |
|---|---|
| R0 | Flags |
| b0 = User mode flag (0=user mode access not required, 1=user mode access required) | |
| b1 = Application space flag (0=context isn’t in application space, 1=context is in application space) | |
| b29 = Context is awaiting lazy activation (1=yes, 0=no) | |
| b30 = Context status registers are active (1=active, 0=saved) | |
| b31 = Memory allocation method (0=user allocated, 1=VFPSupport allocated) | |
| R1 | Number of doubleword registers (may be greater than number requested upon context creation) |
| R2 | Register status. bit ‘n’ is 1 if doubleword register ‘n’ is active, 0 if saved. |
| R3 | Pointer to dump format descriptor block |
| R4 | Context size |
| - | Other registers preserved |
The format descriptor block is a list of words describing the format of the register dump. The list is terminated by the word -1. Each word is split in two halves: bits 0-15 give the type/format of the data, and bits 16-31 give the byte offset of the data, relative to the context pointer (R0 on entry). Not all entries will be present for all contexts.
Format halfword values:
| Value | Meaning | Data length |
|---|---|---|
| 0 | FPSCR register | 4 bytes |
| 1 | FPEXC register | 4 bytes |
| 2 | FPINST register | 4 bytes |
| 3 | FPINST2 register | 4 bytes |
| 4 | FLDMX/FSTMX format word (as produced by FSTMX for R1 registers, VFPv1 only) | 4 bytes |
| 5 | Register dump (from D0 upwards, as saved by FSTMX if VFPv1 or FSTMD if VFPv2D+, or FSTMS if VFPv2xD+) | 8*R1 bytes |
Returns an error if context not found.
VFPSupport_FastAPI
(SWI &58EC5)
| Entry | |
|---|---|
| - | - |
| Exit | |
|---|---|
| R0 | Workspace pointer to pass in R12 |
| R1 | CheckContext function pointer |
| R2 | CreateContext function pointer |
| R3 | DestroyContext function pointer |
| R4 | ChangeContext function pointer |
| - | Other registers preserved |
FastAPI can be used as a fast way of context switching from privileged modes. Calling convention is as per the individual SWIs, with errors being reported in an identical manner (e.g. V set and error block ptr in R0)
VFPSupport_ActiveContext
(SWI &58EC6)
| Entry | |
|---|---|
| - | - |
| Exit | |
|---|---|
| R0 | Currently active context pointer (or pointer to context pending lazy activation) |
VFPSupport_Version
(SWI &58EC7)
| Entry | |
|---|---|
| - | - |
| Exit | |
|---|---|
| R0 | Module version number * 100 |
VFPSupport_Features
(SWI &58EC8)
| Entry | |
|---|---|
| R0 | Reason code: |
| 0 = Return FPSID, MVFR0, MVFR1 values | |
| - | Other reigsters dependent upon reason code |
| Exit |
|---|
| Dependent upon reason code |
VFPSupport_Features 0
| Entry | |
|---|---|
| R0 | 0 |
| Exit | |
|---|---|
| R0 | FPSID value |
| R1 | MVFR0 value |
| R2 | MVFR1 value |
| - | Other registers preserved |
Returns the values of the FPSID, MVFR0, & MVFR1 registers. Note: VFPv1 has no MVFR0/MVFR1 registers, so on such a system the contents of R1 and R2 should be ignored.
Other notes
For all SWIs, a context pointer of 0 represents a null context (i.e. VFP/NEON access disabled).
As long as the context memory wasn’t allocated by VFPSupport, the context can be freely moved, copied, or deleted by user programs as long as VFPSupport_ExamineContext reports that the context is inactive (bits 28 & 29 of R0 clear, and R2 == 0). However be wary of moving a non-application space context into application space and then activating it. Saving/restoring contexts to/from disc is possible, but is not advised if the context is to be loaded on a different machine or with a different version of VFPSupport, as the internal context structure is subject to change between machine types & module versions.
In order to safely deactivate a context in order to allow it to be manually manipulated, all that is required is to perform a non-lazy context switch to another context, or to the null contet.
Note that the version of VFPSupport_ExamineContext used in VFPSupport 0.02 and above is backwards-incompatible with the version used in version 0.01.