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Alto Hardware Manual
SavedCa r ry:
0
SavedPC:
0
SavedAct ive: ·0
PCLOC:
ACTIVE:
ChanMask:
500
453
37
Section 3: Emulator
28
; contains ones for higher priority channels
It is customary (though not essential) to assign interrupt channel priorities such that channel 15 has the
highest priority and channel 1 the lowest. In this case, the
ChanMask
for channel ts interrupt routine will
consist of 15-i one bits right-justified. In any case,
ChanMask
must
contain zero in the bit corresponding to
the· interrupt channel being serviced.
The code below initiates interrupts on the channels corresponding to one bits in ACO. It must disable
interrupts to prevent
ww
from being changed by microcode-initiated interrupts.
CauseInt:
WW:
COM 0 0
DIR
LDA 1 @WW
AND 0 1
ADC 0 1
STA 1 @WW
EIR
452
AC1
~
ACO OR AC1
the interrupt happens after this
If a channel's
ACTIVE
bit is 0 when viewed from non-interrupt level, u1.en u1.e channel is not in use. The
code below searches
ACTIVE
for the highest priority free channel. It is careful not to assign the parity
interrupt channel. It then initializes an interrupt handler on that channel and returns a word with a one
in the bit position of the assigned channel.
It
must not be called from interrupt level.
; enter with ACO
=
the address of the interrupt handler
InitChan:
STA 0 INTHANDLER
FFC:
SUB 1 1
SUBZL 0 0
LDA 2 @ACTIVE
MOVZL 0 0 SZC
JMP fail
INC 1 1
AND# 0 2 SZR
JMP FFC
LDA 2 INTVEC
ADD 1 2
LDA 3 INTHANDLER
STA 3 0 2
AC1
~
0
ACO
~
1
no interrupt channels free.
free?
no.T ry the next one
install handler in INTVEC
LDA 2 @ACTIVE
; turn on the channel
ADD 0 2
; cant carry: equivalent to OR
STA 2 @ACTIVE
; ACO
=
one-bit mask designating the assigned channel
INTVEC:
501
INTHANDLER:
0
; temp
The code below destroys the interrupt channels corresponding to one bits in ACO. It must not be called
from interrupt level.
DestroyInt:
COM 0 0
LDA 1 @ACTIVE
AND 0 1
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