AppleTalk #7
MLIACTV Flag and the IIe Workstation Card

Written by Mark Day & Dan Strnad (November 1989)

This Technical Note describes a problem using the MLIACTV flag with the IIe Workstation Card.

When using the Apple IIe Workstation Card, the MLIACTV flag does not always show that the MLI (or PFI) is active. This inconsistency can cause programs that use the MLIACTV flag to fail when making MLI calls from interrupt routines. Programs can correct for this problem by making all MLI calls through the NewMLI routine listed in this Note and checking the NewMLIActv flag instead of the MLIACTV flag. This approach solves the problem only if all MLI calls, including those made by any interrupt routines, are made through this routine.

The following routine is a replacement for the MLI entry point at $BF00. Programs using this routine can perform a JSR to NewMLI instead, which fixes the problem. Section 6.2.1 of the ProDOS 8 Technical Reference Manual details how programs can cause the MLI to return the their routine rather than the routine that originally called it. For programs using this technique that are also using the routine below, the location below labeled NewCmdAddr replaces CmdAdr ($BF9C). The steps involved in patching the MLI return location still apply, as specified in Section 6.2.1 of the ProDOS 8 Technical Reference.

; MLI patch for Apple II Workstation Card
; by Mark Day
; code shown is compatible with MPW IIGS cross-assembler
; Your program should use the NewMLIActv flag instead of
; MLIACTV ($BF9B), and should JSR NewMLI instead of
; JSR MLI ($BF00).

           machine M6502            ; 6502 code for //e
           longa  off
           longi  off
parmptr    equ    0                 ; two bytes on zero page
MLI        equ    $BF00             ; entry to the real MLI

NewMLI     proc

           php                      ; save old interrupt status to
           pla                      ; temporarily disable interrupts
           sta    oldp              ; so that NewCmdAddr is always valid
           sei                      ; when an interrupting routine sees
                                    ; NewMLI active.

           ror    NewMLIActv        ; NewMLI is now active!

; We need to get the return address from the stack so we can
; get the command number and parameter block address which
; follow the JSR NewMLI, and so we can save NewCmdAddr.
           pla                      ; get low byte of parm address - 1
           sta    parmptr
           adc    #4                ; get real return address
           sta    NewCmdAddr
           sta    parmptr+1         ; save high byte of parm address - 1
           adc    #0
           sta    NewCmdAddr+1      ; save real return address

           lda    oldp
           plp                      ; reinstate old interrupt status
; Now, we copy the call number and parameter list pointer that followed
; the JSR NewMLI, and copy them after a JSR to the real MLI.
           tya                      ; save Y on stack
           ldy    #1                ; offset to command number
           lda    (parmptr),y       ; get command number
           sta    NewCmdNum
           iny                      ; point to parm list ptr (low)
           lda    (parmptr),y
           sta    NewParmPtr
           lda    (parmptr),y
           sta    NewParmPtr+1
           pla                      ; unstack value of y register

; Now, call the real MLI with the user's command and parameter list
; and jump back to our caller.
           jsr    MLI               ; call the real MLI
NewCmdNum  dc.b   0                 ; command number
NewParmPtr dc.w   0                 ; parameter list pointer

           php                      ; save C because LSR changes it!
           lsr    NewMLIActv        ; MLI is no longer active
           plp                      ; restore C
           dc.b   $4C               ; JMP absolute instruction
NewCmdAddr dc.w   0                 ; target of jump, caller's return address

NewMLIActv dc.b   0                 ; $80 bit set if MLI active
oldp       ds.b   1                 ; used to preserve processor status


Note that this routine also works on the Apple IIGS, even though the problem with the MLIACTV flag only affects Apple IIe Workstation Cards.

Further Reference

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