0020-003F ---- PIC 1   (Programmable Interrupt Controller 8259)


0020   w   PIC initialization command word ICW1
        bit 7-5 = 0  only used in 80/85 mode
        bit 4   = 1  ICW1 is being issued
        bit 3   = 0  edge triggered mode
            = 1  level triggered mode
        bit 2   = 0  successive interrupt vectors use 8 bytes
            = 1  successive interrupt vectors use 4 bytes
        bit 1   = 0  cascade mode
            = 1  single mode, no ICW3 needed
        bit 0   = 0  no ICW4 needed
            = 1  ICW4 needed

0021   w   PIC  ICW2,ICW3,ICW4 after ICW1 to 0020
       ICW2:
        bit 7-3 = address lines A0-A3 of base vector address for PIC
        bit 2-0 = reserved
       ICW3:
        bit 7-0 = 0  slave controller not attached to corresponding
                 interrupt pin
            = 1  slave controller attached to corresponding
                 interrupt pin
       ICW4:
        bit 7-5 = 0  reserved
        bit 4   = 0  no special fully-nested mode
            = 1  special fully-nested mode
        bit 3-2 = 0x nonbuffered mode
            = 10 buffered mode/slave
            = 11 buffered mode/master
        bit 1   = 0  normal EOI
            = 1  Auto EOI
        bit 0   = 0  8085 mode
            = 1  8086/8088 mode

0021   r/w PIC master interrupt mask register
       OCW1:
        bit 7 = 0  enable parallel printer interrupt
        bit 6 = 0  enable diskette interrupt
        bit 5 = 0  enable fixed disk interrupt
        bit 4 = 0  enable serial port 1 interrupt
        bit 3 = 0  enable serial port 2 interrupt
        bit 2 = 0  enable video interrupt
        bit 1 = 0  enable keyboard, mouse, RTC interrupt
        bit 0 = 0  enable timer interrupt
       OCW2:
        bit 7-5 = 000  rotate in auto EOI mode (clear)
            = 001  nonspecific EOI
            = 010  no operation
            = 011  specific EOI
            = 100  rotate in auto EOI mode (set)
            = 101  rotate on nonspecific EOI command
            = 110  set priority command
            = 111  rotate on specific EOI command
        bit 4   = 0    reserved
        bit 3   = 0    reserved
        bit 2-0    interrupt request to which the command applies

0020   r   PIC  interrupt request/in-service registers by OCW3
       request register:
        bit 7-0 = 0  no active request for the corresponding int. line
            = 1  active request for corresponding interrupt line
       in-service register:
        bit 7-0 = 0  corresponding line not currently being serviced
            = 1  corresponding int. line currently being serviced

0020   w   PIC OCW3
        bit 7   = 0   reserved
        bit 6-5 = 0x  no operation
            = 10  reset special mask
            = 11  set special mask
        bit 4   = 0   reserved
        bit 3   = 1   reserved
        bit 2   = 0   no poll command
            = 1   poll command
        bit 1-0 = 0x  no operation
            = 10  read int.request register on next read at 0020
            = 11  read int.in-service register on next read 0020