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Subject: File: "MINIX-L MAIL" being sent to you
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Date: Thu, 1-Oct-87 23:33:09 EDT
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Posted: Thu Oct  1 23:33:09 1987
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Sender:       Minix operating system <MINIX-L@NDSUVM1>
From:         Marty Leisner <martyl@rocksvax.uucp>
Subject:      Controlling processer status -- Minix v. Xinu
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To: $PEER$ <MINIX-L@FINHUTC>

Minix uses two routines -- lock()/restore() which saves the processor
status in a common private variable (lockvar).

If found this way of coding to often leads to problems when you want to
nest subroutines which each require the interrupts be turned off,
and you don't want to write the code assuming the low-level subroutine is
running with the interrupts turned off.

Xinu uses two subroutines -- disable()/restore().  Disable returns the
processor status and restore gets passed the new processor status -- the
psw should be stored in local memory of the calling function.

I've used variants of the lock/restore/unlock variant for several years and
have gotten bitten too many times than I care to remember.  I'm going to
change all references of lock/restore to disable/restore (I just got bitten
again doing some hacking).

Any comments?

Here's the code for disable/restore (real simple):

; disable interrupts and return previous flags setting
procdef disable
    pushf
    cli
    pop    ax    ; to return to caller
    pret
pend     disable

; restore processor status as it was
procdef restore, <<proc_status, word>>
        push    proc_status
        popf            ; flags are according to proc_status
        pret
    pend    restore

P.S.  The above was written using the Aztec assembler (it has a series of
macros which knows how to get at arguments.  If arguments are passed in, it
knows to do a
    push    bp
     mov    bp,sp

and pret does a
    pop    bp

only if bp was pushed).

marty



--
marty leisner
xerox corp
leisner.henr@xerox.com
martyl@rocksvax.uucp