IRQ

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Joined: Aug 12 2015
Posts: 149

I am doing some research to find out more information about
how IRQ's work/operate.

Can someone help that can read schematics.

Here is the link.

https://apple2.gs/downloads/Apple_IIgs_256K_Schematic.pdf

On the CPU, the IRQ is lead to a box that contains the following

IRQ L, 2,3,4,7,5,8

What are these numbers? (and the L for that matter)

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speedyG's picture
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Joined: Nov 16 2011
Posts: 2376
Re: IRQ

wishtoknow wrote:

Can someone help that can read schematics.
On the CPU, the IRQ is lead to a box that contains the following
IRQ L, 2,3,4,7,5,8
What are these numbers? (and the L for that matter)

"L" means active "low" and the numbers indicate at which pages the signalline is continued....
speedyG

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In early days I had a lot of money but no time - now I have no money but a lot of time....
the second part includes less friends but a lot more joy on life....

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Joined: Aug 12 2015
Posts: 149
Re: IRQ

speedyG

Thanks.

So

2. Leads to Mega II
3. Leads to VGC
4. Z80 SCC
5. KeyGloo
7. Slots
8. DOC5503 - Ensoniq Chip

I am interested in 2,3,8.

So for the Mega II, how does the timing of the chip that
generates the IRQ. Is it other parts of the chip that does this?
And I guess for the rest?
I noticed for the VGC, it also has an EXTIRQ (Pin 40) that leads
to TP93. (What is that?)

speedyG's picture
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Posts: 2376
Re: IRQ

Hello wishtoknow,

the precise answer to this exceeds far the limits of a posting....
you could write a entire book about that topic......
I'll try to cut down the answer to a frank and simple answer here at the moment...

In general the IRQ (Interrupt Request ) handles the handshaking between the Interfacecards ( like serial comm,
printer etc. - i.e. peripherial devices ) and the CPU at the motherboard ( in more complex situation the CPU
on another CPU-Interface card like Z80 in CPM or 8086 at the PC-Transporter card or the 6809 at the Stellation Mill in OS9 ).

relation to MEGAII chip and the Slots:
It is related to the INT ( Interrupt signal and several other signals ( to get syncronized with them ).

while the INT signal is related to a daisy chain to handle priority of interfacecards from top priority of slot 7 down to
lowest priority of slot 1 ( i.e. external devices ) or internal devices like sound or joystick.

related to VGC-chip and the MEGA II chip:
Second you must bear in mind that a general cycle of the computer is split apart in 2 halfcycles
( one half dedicated to the CPU - i.e. "brain of the computer ) and the other cyclehalf dedicated to the
videodisplay of the contents in Memory in the RAM. During this aecond halfcyle the INT and INTREQ will be
ignored and have to "shut up" while display is performed - so they can "act" only in the first halfpart of
the cycle.

related to the DOC5503 chip / Ensoniq Chip:
The sound is just handled similar to the access of the joystick like another peripherial devices or CPU-cards .......

and there is a further close dependency to the so called "soft switches" ( specific memory locations )
that act like switches ( bit is 1 equals to "ON" or bit is = 0 equals to "OFF" ).....
this softswitches are used by software ( either in program or from code in Eprom of the interfacecard and pass
signal to or from CPU and to or from Interface that Data is to be picked up from Databus or that pickup is completed.....
INT signals demand for Pickup - and IRQ signals that either Data is availiable or pickup is completed...

2 things to bear in mind:
a) there was a development from Apple II where that has been handled by a bunch of logic chips
b) to the IIe where that bnch of logic-chips have been compacted to the MMU chip and the IOU chip as well as the HAL chip
c) and finaly the largest integration at the IIGS crunching the Handling down to the MEGAII chip and the VGCchip
keeping besides the HAL-chip for the more basical cycle-timing.
and seond the handling of "switched memory" caused by the adressing limits of the Apple II series to 64kB
and the 128 kB limitation at the IIe and finally the 16 MB at the IIGS - switching portions of the expanded video-memory to the
lower 64 kB portion of memory for the Videodisplay....which is also related to the handling of Interupts...

therefor i urgently recomend to read first the books:
JIM SATHER - understanding the Apple II or WINSTON GAYLOR - the Apple II circuit description
then continue with:
JIM SATHER - understanding the IIe
to understand the topic itself and the related topics...

the Keygloo is just another chip for communication with the kayboard acting also as peripherial device like sound or joystick...

don't forget: this is just a very compacted explenation.....
sincerely
speedyG

TP93 is just for measuring...

__________________

In early days I had a lot of money but no time - now I have no money but a lot of time....
the second part includes less friends but a lot more joy on life....

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Joined: Aug 12 2015
Posts: 149
Re: IRQ

Again speedyG thanks for the reply.

the precise answer to this exceeds far the limits of a posting....
you could write a entire book about that topic......
I'll try to cut down the answer to a frank and simple answer here at the moment

Are there books on this topic? If so could you direct me to them.

relation to MEGAII chip and the Slots:
It is related to the INT ( Interrupt signal and several other signals ( to get synchronized with them ).

What are the other signals? And how does it get synchronized. This is the information I am
really looking after.

therefor i urgently recomend to read first the books:
JIM SATHER - understanding the Apple II or WINSTON GAYLOR - the Apple II circuit description
then continue with:
JIM SATHER - understanding the IIe
to understand the topic itself and the related topics...

I have read these books along time ago, I can re-read if necessary, but I am looking at
programming the IIGS in native mode and using the new features. It offers. I am not interested
in the II emulation mode. So I think that these books will apply. (Correct me if I am wrong.)