On Mon, May 11, 2020 at 09:41:27AM -0400, James Knott via talk wrote:
I used to be a computer tech, on the big systems. I also designed and built an 8 serial port card for my IMSAI 8080. In all my experience, I had only seen level triggered interupts or IRQs. This meant when something needed service, it would pull the signal line low. This allowed multiple devices to share 1 interrupt line and the OS would sort out which needed to be serviced. IBM, instead of using that common practice, when with rising edge triggers, which cannot be easily shared. The Intel interrupt controller chip that was used supported either mode, so there was no valid hardware reason, that I could see, for IBM to choose that method. If they had gone with level trigger, then we wouldn't have had that IRQ sharing mess we had to deal with on the PC/AT bus.
I wonder if edge triggered allowed for dumber devices since the device doesn't need to be able to latch the interrupt until serviced. I could imagine anything that could make the IBM PC cheaper was considered worth doing (like the 8 vs 16 bit data bus by using the 8088 rather than the 8086). But yes edge triggered interrupts are awful. -- Len Sorensen