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HP OpenVMS Systemsask the wizard |
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The Question is: Hi there Mr Wizard! I have a question regarding accessing the general purpose registers from C, which I hope you can help me with. The short version: I'm running DEC C V6.0-001 on OpenVMS Alpha V7.3.1 and need to read and write the general purpose registers. The documentation talks extensively about the _READ_GPR and _WRITE_GPR, but they don't seem to be available on the Alpha (in builtins.h, they're specifically excluded with an #ifdefined(__VAX) conditional). Can you tell me what the alternative is? The long version: I need to write a function which provides the caller with the initial buffer allocation (total size) for a mailbox. From what I can determine, the only place this is stored is in ucb$w_iniquo, to which I can find no documented interface. It seems that the best way for me to get the UCB is to call ioc$verifychan to get the CCB, and then follow the UCB link from there (ccb$l_ucb). The documentation for IOC$VERIFYCHAN indicates that it wants the channel number in R0, and will return the CCB in R1. Is there a supported method for accessing these registers from C? Is there some arcane operation involving, perhaps, the linkage pragmas that will enable me to call IOC$VERIFYCHAN without poking the registers directly? Many thanks, Steve Lock. The Answer is : The asm directives are the usual approach for writing assembler code using the C compiler. In general, writing assembler in C, and directly reading and writing the underlying hardware registers is discouraged. The linkage pragma is the usual approach for defining non-standard calling interfaces, of course. Use of a standard calling interface such as that of IOC_STD$VERIFYCHAN would be preferable to the use of a non-standard calling interface such as that of IOC$VERIFYCHAN, too. There are changes to UCB$W_INIQUO and such underway; the field is being increased to the longword UCB$L_MB_INIQUO cell. The best and most portable and upgradable solution -- kernel-mode code can require periodic recompilation and relinking, as OpenVMS is upgraded or ECOs are applied -- would be to correctly set and to then remember the value when the mailbox is initially created.
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