Details of Sys V/AT interrupt request enabling (long)

John Kennedy johnk at opel.uu.net
Mon Jan 15 02:47:49 AEST 1990


This is a description of the way interrupts are masked out and
enabled in Sys V/AT.  This posting is as a result of mail I've
received requesting this information.

This is the way I have come to understand the mask tables.  This
information may not be infallible, so temper it with your own
experiences.

Thanks to mike at kc5n.UUCP (Mike Barnes), for information that
led me to understand how this all works.

_____

Whenever the system wishes to enable or disable a group of interrupts,
it does so by entering a new spl() level.  In V/AT, the levels that
are used are 0, 4, 5, 6 and 7.  Each level has associated with it
an interrupt mask which is used to disable and enable interrupts upon 
entering that and leaving that level.  The specific interrupts are
designated by two arrays, named 

mastbl and slavtbl,

corresponding to the masks to be applied to the master interrupt controller
(PIC) and slave PIC, respectively.  

The default settings are:

     mastbl[0]    [1]    [2]    [3]    [4]    [5]    [6]    [7]
            00     xx     xx     xx     41     E1     E3     FF

    slavtbl[0]    [1]    [2]    [3]    [4]    [5]    [6]    [7]
            00     xx     xx     xx     61     61     61     61

The xx's represent a don't care value, as levels 1,2 and 3 are not used (to
the best of my knowledge).

For each byte of either table, a 1 indicates that an interrupt is disabled,
and a 0 represents that the interrupt is enabled.

On the master interrupt controller, the interrupts are allocated as:

     bit [0 = low order bit]    bit
     0  IRQ0 (Clock)            4  IRQ4 Serial (tty0)
     1  IRQ1 Console Keyboard   5  IRQ5 Parallel Port 2
     2  IRQ2 Slave PIC          6  IRQ6 Floppy Controller
     3  IRQ3 Serial (tty1)      7  IRQ7 Parallel Port 1

On the slave interrupt controller, the interrupts are allocated as:

     bit                        bit
     0  IRQ8  Real time clock   4  IRQ12 Spare
     1  IRQ9  PC/XT IRQ2        5  IRQ13 Math Coprocessor
     2  IRQ10 Spare             6  IRQ14 Spare Hard Disk Controller
     3  IRQ11 Spare             7  IRQ15 Spare

>From the 'Adding Devices Drivers' section of TFM, we see that Microport
allocates each of the spl() levels as follows:

SPL Value     Interrupt(s)      Disables                   mastbl      slavtbl
              masked            interrupts on:

spl0          none              none                           00      01
spl4          0,6,8,13,14       clock, floppy disk,            41      61
                                math coproc., hard disk
spl5          0,6,8,13,14,5,7   above plus parallel ports      E1      61
spl6          0,6,8,13,14,5,7,1 above plus keyboard            E3      61
spl7          0,6,8,13,14,      above plus serial ports        FF      61
              5,7,1,3,4,2       and slave PIC

The kernel (/unix.lk.hd) that was shipped with my 2.4.0L system had these
locations set to:

SPL Value     Interrupt(s)                                 mastbl      slavtbl
              masked            

spl0          8,9,10,11,12                                     00      1F
spl4          0,6,8,9,10,11,12,13,14                           41      7F
spl5          0,6,5,7,8,9,10,11,12,13,14                       E1      7F
spl6          0,6,5,7,1,8,9,10,11,12,13,14                     E3      7F
spl7          0,6,5,7,1,3,4,2,8,9,10,11,12,13,14,15            FF      FF

This is contrary to the documentation, but shows that Microport had
liked to keep the unused IRQs 9,10, 11 and 12 disabled some of the time
and disable IRQ15 with the serial ports.  This is and should be of no
consequence, as Microport didn't use the interrupts on the slave PIC.

Now, let's assume that you've build a new kernel that uses 5 serial ports.
Let's assume that tty0, tty1, tty2, tty3, and tty4 use interrupts
4, 3, 7, 9, and 10, respectively. 

Let's assume that our serial card has the capability to access these interrupts.
In my personal case, I have cut and jumpered cards to use new interrupts.
In the case of an 8-bit serial card needing to access IRQs 8-15, jumpers
can run to a nearby memory card, or any other containing the extended bus
connectors.

Notice that tty2 uses IRQ7, which is normally used by the first parallel port.
In this example, let's assume the system has no parallel ports.  Thus, the
interrupt is available for use, but must be enabled when the other serial ports
are enabled, and not with the parallel ports.  The new, previously spare
interrupts must be enabled with the serial interrupts.  The new values
for the master and slave tables look like:

SPL Value     Interrupt(s)      Disables                   mastbl      slavtbl
              masked            interrupts on:

spl0          none              none                           00      01
spl4          0,6,8,13,14       clock, floppy disk,            41      61
                                math coproc., hard disk
spl5          0,6,8,13,14       *above plus parallel ports     41      61
spl6          0,6,8,13,14,1     above plus keyboard            43      61
spl7          0,6,8,13,14,1,
              4,3,7,9,10,2      above plus serial ports        DF      67
                                and slave PIC

*(no change, as no parallel ports exist in this example)

/etc/patch is used to change the values.  The kernel is patched for the above
example by:

/etc/patch kernelname -b +0 mastbl  0x00    # nothing
/etc/patch kernelname -b +0 slavtbl 0x01    # clock timer

/etc/patch kernelname -b +4 mastbl  0x41    # + floppy, clock
/etc/patch kernelname -b +4 slavtbl 0x61    # + winch, math

/etc/patch kernelname -b +5 mastbl  0x41    # + parallel (none)
/etc/patch kernelname -b +5 slavtbl 0x61    # 

/etc/patch kernelname -b +6 mastbl  0x43    # + keyboard
/etc/patch kernelname -b +6 slavtbl 0x61    #

/etc/patch kernelname -b +7 mastbl  0xdf    # + slave pic & int 3, 4, 7 (serial)
/etc/patch kernelname -b +7 slavtbl 0x67    # + int 9 & 10

Other combinations are left as an exercise for the reader :-).

Before building the kernel, don't forget that /usr/linkkit/cf/master
must be edited to reflect what device uses each interrupt and the
total count of each kind of device, and /usr/linkkit/cf/dfile.wini
must be edited to reflect the total count of each kind of device.

Don't forget, too, that any card that allows IRQ2 gets that interrupt 
serviced by IRQ9 on the software end.  The bus line for IRQ2 is rerouted to 
IRQ9 on the slave PIC, and IRQ2 is unusable on the AT bus.

John

-- 
John Kennedy                     johnk at opel.uu.uunet
Second Source, Inc.
Annapolis, MD



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