Nick de Smith, 09-Mar-88

AST notes:

AST type		Call	IPL		ACB deallocated		ACB$B_MODE		AST address/parameter
--------------		----	---		---------------		----------		---------------------
Special KERNEL		JSB	IPL$_ASTDEL(3)	No (1)			ACB$V_KAST (5)		ACB$L_KAST	n/a

Piggy-Back-KAST (6)	JSB	IPL$_ASTDEL(4)	No (1)			ACB$V_PKAST		ACB$L_KAST	n/a

KERNEL,SUPER,EXEC,USER	CALLG	0		Yes (1) (2)		PSL$C_USER etc.		ACB$L_AST  / ACB$L_ASTPRM
	(call mechanism/parameters as per $DCLAST)

Notes:

(1) Where the ACB is not deallocated by the system, it is the responsibility of the AST routine to free it, otherwise
    non-paged pool will be consumed. Note that you can read most system data areas from EXEC mode, but you can't write
    to them. Therefore you can only deallocate an ACB at KERNEL mode.

(2) The ACB is deallocated before the AST routine is called unless ACB$V_PKAST or ACB$V_NODELETE was set in ACB$B_MODE,
    in which case it is the ASTs responsibility to deallocate the ACB (see (1) above).

(3) Special KERNEL mode ASTs can drop and raise IPL as the calling routine (SCH$ASTDEL) resets it for you.

(4) PKASTs can drop and raise IPL, but must exit with IPL$_ASTDEL.

(5) If ACB$V_KAST is set, only a special KERNEL mode AST will be delivered. No other bits in ACB$B_MODE are used.

(6) PKASTs are called before the associated "normal" AST. There must be both an ACB$L_KAST and a ACB$L_AST address
    specified if ACB$V_PKAST is set.

To queue an AST to another process:

		movl	#ACB$K_LENGTH, r1	; Set size of ACB
		jsb	g^EXE$ALONONPAGED	; Allocate nonpaged pool space for normal ACB
		movl	r2, r5			; R5 => New ACB
		movb	#DYN$C_ACB, ACB$B_TYPE(r5) ; Say block is an ACB
		movw	r1, ACB$W_SIZE(r5)	; Store size of structure	
		movl	#<ipid>, ACB$L_PID(r5)	; Store IPID of target process (could be our IPID)
		movb	#<flags>, ACB$B_RMOD(r5) ; Store type of AST in ACB
		movab	<ast_code>, ACB$L_xAST(r5) ; Set the AST address
		movl	#PRI$_xxx, r2		; SCH$QAST needs priority boost in R2
		jsb	g^SCH$QAST		; Queue the AST

This piece of code is simplistic. It would probably work in some/most situations. There again, it will crash
your system in some situations too! See K_AST.MAR for a better (definitive?) treatment.

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