8088 interrupts

1
8088 interrupts

• Interrupts are the most common way for external
  hardware to signal the microprocessor that some
  kind of service is needed (such as reading a key
  from the keyboard)
• There are 256 different types of 8088 interrupts
  (00 - 0FFh)
• Interrupts can be caused by
• internal interrupts (exceptions)
• external events (NMI and INTR)
• software instructions (INT xx)

2
8088 interrupt sequence

• When the 8088 recognizes an interrupt, it
• 1) Completes execution of the current instruction
• 2) Pushes the flag register onto the stack
• 3) Clears the interrupt enable bit
• 4) Pushes the CS of the next instruction onto
  the stack
• 5) Pushes the IP of the next instruction onto
  the stack
• 6) Reads the interrupt type from the data bus
• 7) Does a Jump Indirect to the address stored
  at
• type4, type4 1 IP of interrupt service
  routine type4 2, type4 3 CS of interrupt
  service routine
• Note this implies that the lowest 1K of memory
  is reserved to hold the interrupt vector table.

3
Interrupt service routines

• An interrupt service routine is simply an
  assembly language program that handles the device
  requesting an interrupt.
• An interrupt service routine is terminated by an
  IRET instruction that
• 1) pops the CS of the next instruction from the
  stack
• 2) pops the IP of the next instruction from the
  stack
• 3) pops the flag register from the stack

What function must the interrupt service routine
perform? Cause the INTR line to go low!
4
NMI

• The NMI (Non-maskable interrupt) is a
  positive-edge-sensitive external input to the
  8088
• The NMI cannot be ignored by the 8088
• An NMI is a type 2 interrupt.
• Its interrupt vector is stored at addresses 08,
  09, 0A, and 0B.
• In order to be recognized as a positive edge, the
  NMI line must be low for two clock cycles
• The external device must hold NMI high after
  causing the edge

5
INTR

• INTR is an active-high level-sensitive interrupt
  request line (external 8088 input)
• Interrupt requests are recognized if the
  interrupt enable bit is 1
• Interrupt requests are ignored if the interrupt
  enable bit is 0
• SEI sets the interrupt enable bitCLI clears the
  interrupt enable bit

6
INTA

• INTA in the 8088s interrupt acknowledge pin (an
  external output)
• When the 8088 recognizes an interrupt, it sends a
  low pulse on INTA to signal the external device
  to get ready to put the interrupt type on the
  data lines
• A second low pulse on INTA signals the external
  device to put the type number on the data lines

7
Interrupt priorities

• An interrupt service routine can itself be
  interrupted only by a higher priority interrupt
• Priorities (from highest to lowest)
• Reset
• Internal interrupts and exceptions
• Software interrupts
• Non-maskable interrupt
• External hardware interrupts
• For external hardware interrupts, the higher the
  type number, the lower the priority

8
Interrupt types

• Type 0 Divide Error
• Type 1 Single step
• Type 2 NMI
• Type 3 Breakpoint
• Type 4 Overflow
• Type 510 - 3110 Reserved (internal software and
  BIOS)
• Type 3210 - 25510 User available

9
Homework for Thursday, November 4

• Draw the connections between the 8255s and write
  8088 assembly language programs to have two
  computers communicate using 8255s.
• Computer 1 should transmit using PortB as an
  output in Mode 1. Transmit 20 bytes of data from
  an array called DATAOUT (DATAOUT db
  ?,?,?,?,?,?,?,?)
• Computer 2 should receive using PortA as an input
  in Mode 1. Store the 20 bytes received in an
  array called DATAIN (DATAIN db ?,?,?,?,?,?,?,?)