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Apple II
Technical Notes
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Developer Technical Support
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.
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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.
sec
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.
;
clc
pla ; get low byte of parm address - 1
sta parmptr
adc #4 ; get real return address
sta NewCmdAddr
pla
sta parmptr+1 ; save high byte of parm address - 1
adc #0
sta NewCmdAddr+1 ; save real return address
lda oldp
pha
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
pha
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
iny
lda (parmptr),y
sta NewParmPtr+1
pla ; unstack value of y register
tay
;
; 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
endp
end
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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o AppleShare Programmer's Guide for the Apple IIGS
o ProDOS 8 Technical Reference Manual