Macintosh IIfx VIA Registers

 

VIA Registers*

VIA1 Data Register A
Bit	Direction	Bit Name	Description
6	Input	CPU.ID3	Bit 3 of 4 bit model identity code1
4	Input	CPU.ID2	Bit 2 of 4 bit model identity code1
2	Input	CPU.ID1	Bit 1 of 4 bit model identity code1
1	Input	CPU.ID0	Bit 0 of 4 bit model identity code1
VIA1 Data Register B
7	Output	vSndEnb	0 = Sound Enable (for software compatability)
2	Output	rTCEnb	0 = Real Time Clock enabled
1	Output	rtcCLK	RTC's data clock line
0	Input/Output	rtcData	RTC's serial data line
1- The CPUID the Macintosh IIfx has a value of 1101.

VIA2 Data Register A
Bit	Direction	Bit Name	Description
7	Output	v2RAM1	RAM-size bit 1
6	Output	v2RAM0	RAM-size bit 0
5	Input	v2IRQ6	Interrupt request from expansion slot $E
4	Input	v2IRQ5	Interrupt request from expansion slot $D
3	Input	v2IRQ4	Interrupt request from expansion slot $C
2	Input	v2IRQ3	Interrupt request from expansion slot $B
1	Input	v2IRQ2	Interrupt request from expansion slot $A
0	Input	v2IRQ1	Interrupt request from expansion slot $9
VIA2 Data Register B
7	Output	v2VBL	60.15 Hz interrupt request to VIA1
7	Input	v2SNDEXT	0 = plug is inserted in external sound jack
5	Input	v2TM0A	Transfer mode acknowledge bit 0 from NuBus
4	Input	v2TM1A	Transfer mode acknowledge bit 1 from NuBus
2	Output	v2PowerOff	0 = shut power off
1	Output	v2BusLk	0 = NuBus transactions are locke out
0	Output	v2CDis	0 = disable instruction/data cache

* Information gathered from Guide to Macintosh Family Hardware.

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