From: byron@cc.gatech.edu (Byron A Jeff) Subject: Re: Serial: My IRQ is 5 NOT 4! Change? Date: Sat, 21 Aug 1993 15:26:28 GMT
In article <745772049snz@wise.demon.co.uk>,
Simon Wise <simon@wise.demon.co.uk> wrote:
>In article <1993Aug17.203434.163@dg8lav.toppoint.de> kalle@dg8lav.toppoint.de writes:
>
>>Greg Corteville (gcortevi@nyx.cs.du.edu) wrote:
>>: I purposely installed COM 3 on my system to have IRQ 5 so that I could use
>>: COM 1 and COM 3 on my system at the same time without problems. However,
>>: when Linux loads, it thinks its some kind of error and defaults to IRQ 4
>>: on COM 3. How can I force it to use IRQ 5?
>>
>>
>>Use SETSERIAL Version 2.x. Linux can't install the serial ports at the boot-up.
>>You will find setserial on all servers. (i hope so :-)
>>
>>Kalle
>>
>
>I have 4 Serial ports using IRQ 3,4,7 & 9. Using Setserial auto-irq or
>manual the port on IRQ9 is changed to IRQ2. Anyone know why ?.
Sure. The original PC had 8 interrupts. It used the 8259 interrupt
controller that has 8 inputs. When the AT and the ISA bus was released 8
more interrupts were added by cascading a second 8259 to the first. But
to do that one of the original interrupt lines had to be replaced. INT 2
was chosen. But since INT 2 was on the original bus and still needed to
exist it was moved to the INT 9 slot. So on the ISA bus INT 2 replaces INT 9.
Quick picture:
Old 8 bit PC Bus
INT 0 -->|----|
... |8259| -> 8088
INT 7 -->|----|
ISA PC Bus
INT 8 -->|----| INT 0 -->|----|
INT 2 -->|8259|-->INT 2 -->|8259| -> 80[234]86
INT 10-->| | ..... -->| |
..... -->| | ..... -->| |
INT 15-->|----| INT 7 -->|----|
Hope this helps,
BAJ