Dr.
Subhasish Mahapatra
Sr. Assistant Professor
School of Electronics Engineering (SENSE)
VIT-AP University
E-Mail: subhasish.m@vitap.ac.in
CONTENT
Interrupt
Dr. Subhasish Mahapatra
Sr. Assistant Professor, SENSE
8085 and 8086 Microprocessors 2
Dr. Subhasish Mahapatra
Sr. Assistant Professor, SENSE
8085 and 8086 Microprocessors 3
INTERRUPT
Interrupt is a process where an external device can get the attention of the microprocessor.
The process starts from the I/O device
The process is asynchronous
Interrupts can be classified into two types:
Maskable (can be delayed)
Non-Maskable (can not be delayed)
Interrupts can also be classified into:
Vectored (the address of the service routine is hard-wired)
Non-vectored (the address of the service routine needs to be supplied externally)
Dr. Subhasish Mahapatra
Sr. Assistant Professor, SENSE
8085 and 8086 Microprocessors 4
INTERRUPT
The 8085 has 5 interrupt inputs
INTR input
Interrupt
The INTR input is the only non-vectored interrupt Maskable Vectored
name
INTR is maskable using the EI/DI instruction pair INTR Yes No
RST 5.5, RST 6.5, RST 7.5 are all automatically RST 5.5 Yes Yes
RST 6.5 Yes Yes
vectored
RST 7.5 Yes Yes
RST 5.5, RST 6.5, and RST 7.5 are all maskable TRAP No Yes
TRAP is the only non-maskable interrupt in the
8085
TRAP is also automatically vectored
Dr. Subhasish Mahapatra
Sr. Assistant Professor, SENSE
8085 and 8086 Microprocessors 5
INTERRUPT
When the Microprocessor receives an interrupt signal, it suspends the currently executing
program and jumps to an Interrupt Service Routine (ISR) to respond to the incoming
interrupt.
Each interrupt will most probably have its own ISR.
Responding to an interrupt may be immediate or delayed depending on whether the
interrupt is maskable or non-maskable and whether interrupts are being masked or not.
There are two ways of redirecting the execution to the ISR depending on whether the
interrupt is vectored or non-vectored.
The vector is already known to the Microprocessor
The device will have to supply the vector to the Microprocessor
Dr. Subhasish Mahapatra
Sr. Assistant Professor, SENSE
8085 and 8086 Microprocessors 6
INTERRUPT
Interrupt Vectors and the Vector Table
An interrupt vector is a pointer to where the ISR is stored in memory.
All interrupts (vectored or otherwise) are mapped onto a memory area called the Interrupt
Vector Table (IVT).
The IVT is usually located in memory page 00 (0000H- 00FFH).
The purpose of the IVT is to hold the vectors that redirect the microprocessor to the
right place when an interrupt arrives.
The IVT is divided into several blocks. Each block is used by one of the interrupts to hold
its “vector”
Dr. Subhasish Mahapatra
Sr. Assistant Professor, SENSE
8085 and 8086 Microprocessors 7
INTERRUPT
Non-Vectored Interrupt Process
1. The interrupt process should be enabled using the EI instruction.
2. The 8085 checks for an interrupt during the execution of every instruction.
3. If there is an interrupt, the microprocessor will complete the executing instruction, and start a
RESTART sequence.
4. The RESTART sequence resets the interrupt flip flop and activates the interrupt acknowledge signal
(INTA).
5. Upon receiving the INTA signal, the interrupting device is expected to return the op-code of one of the 8
RST instructions.
6. When the microprocessor executes the RST instruction received from the device, it saves the address of
the next instruction on the stack and jumps to the appropriate entry in the Interrupt Vector Table (IVT).
Dr. Subhasish Mahapatra
Sr. Assistant Professor, SENSE
8085 and 8086 Microprocessors 8
INTERRUPT
Non-Vectored Interrupt Process
6. The IVT entry must redirect the microprocessor to the actual service
routine.
7. The service routine must include the instruction EI to re-enable the
interrupt process.
8. At the end of the service routine, the RET instruction returns
the execution to where the program was interrupted.
The 8085 recognizes 8 RESTART instructions: RST0 - RST7.
each of these would send the execution to a predetermined hard-
wired memory location
Dr. Subhasish Mahapatra
Sr. Assistant Professor, SENSE
8085 and 8086 Microprocessors 9
INTERRUPT
Multiple Interrupts & Priorities
The microprocessor can only respond to one signal on INTR at a time.
Therefore, the signal from only one of the devices is allowed to reach the microprocessor.
It is required to assign some priority to the different devices and allow their signals to reach
the microprocessor according to the priority.
The solution is to use a circuit called the priority encoder (74366).
This circuit has 8 inputs and 3 outputs.
The inputs are assigned increasing priorities according to the increasing index of the input.
Input 7 has highest priority and input 0 has the lowest.
The 3 outputs carry the index of the highest priority active input.
Dr. Subhasish Mahapatra
Sr. Assistant Professor, SENSE
8085 and 8086 Microprocessors 10
INTERRUPT
Multiple Interrupts & Priorities
Dr. Subhasish Mahapatra
Sr. Assistant Professor, SENSE
8085 and 8086 Microprocessors 11
INTERRUPT
8085 Maskable/Vectored Interrupts
The 8085 has 3 Masked/Vectored interrupt inputs: RST 5.5, RST 6.5, RST 7.5
They are all maskable.
They are automatically vectored according to the following table:
The vectors for these interrupts fall in between the vectors of RST instructions. That is why, the
name is RST 5.5 (between RST 5 and RST 6).
RST 7.5 is positive edge sensitive. RST 6.5 and RST 5.5 are level sensitive.
Dr. Subhasish Mahapatra
Sr. Assistant Professor, SENSE
8085 and 8086 Microprocessors 12
INTERRUPT
RST 5.5, RST 6.5, RST 7.5
These three interrupts are masked at two levels:
Through the Interrupt Enable flip flop and
the EI/DI instructions.
The interrupt Enable flip flop controls the
whole maskable interrupt process.
Through individual mask flip flops that
control the availability of the individual
interrupts.
These flip flops control the interrupts
individually.
Dr. Subhasish Mahapatra
Sr. Assistant Professor, SENSE
8085 and 8086 Microprocessors 13
INTERRUPT
8085 Maskable/Vectored Interrupt Process
1. The interrupt process should be enabled using the EI instruction.
2. The 8085 checks for an interrupt during the execution of every instruction.
3. If there is an interrupt, and if the interrupt is enabled using the interrupt mask, the microprocessor
will complete the executing instruction, and reset the interrupt flip flop.
4. The microprocessor then executes a call instruction that sends the execution to the appropriate
location in the interrupt vector table.
5. When the microprocessor executes the call instruction, it saves the address of the next instruction
6. on the stack and jumps to the specific service routine. The service routine must include EI
instruction to re-enable the interrupt process.
7. At the end of the service routine, the RET instruction returns the execution where the program was
interrupted.
Dr. Subhasish Mahapatra
Sr. Assistant Professor, SENSE
8085 and 8086 Microprocessors 14
INTERRUPT
SIM (Set Interrupt Mask) Instruction
Dr. Subhasish Mahapatra
Sr. Assistant Professor, SENSE
8085 and 8086 Microprocessors 15
INTERRUPT
Example: Set the interrupt masks so that RST 5.5 is enabled, RST 6.5 is masked, and RST 7.5 is
enabled.
First, determine the contents of the accumulator
Dr. Subhasish Mahapatra
Sr. Assistant Professor, SENSE
8085 and 8086 Microprocessors 16
INTERRUPT
RIM Instruction
Dr. Subhasish Mahapatra
Sr. Assistant Professor, SENSE
8085 and 8086 Microprocessors 17
INTERRUPT
RIM Instruction
Since the 8085 has five interrupt lines, interrupts may occur during an ISR and remain
pending.
Using the RIM instruction, the programmer can read the status of the interrupt lines and
find if there are any pending interrupts.
The advantage is being able to find about interrupts on RST 7.5, RST 6.5, and RST 5.5 without
having to enable low level interrupts like INTR.
Dr. Subhasish Mahapatra
Sr. Assistant Professor, SENSE
8085 and 8086 Microprocessors 18
INTERRUPT
RIM Instruction:
Read Interrupt Mask
Load the accumulator with an 8-bit pattern showing the status of each interrupt pin and mask.
Dr. Subhasish Mahapatra
Sr. Assistant Professor, SENSE
8085 and 8086 Microprocessors 19
INTERRUPT
Example: Set the mask to enable RST 6.5 without modifying the masks for RST 5.5 and RST 7.5.
Dr. Subhasish Mahapatra
Sr. Assistant Professor, SENSE
8085 and 8086 Microprocessors 20
INTERRUPT
TRAP
TRAP is the only non-maskable Interrupt.
It does not need to be enabled because it can not be disabled.
It has the highest priority amongst interrupts.
It is edge and level sensitive.
It needs to be high and stay high to be recognized.
Once it is recognized, it won't be recognized again until it goes low, then high again.
TRAP is usually used for power failure and emergency shutoff.
Dr. Subhasish Mahapatra
Sr. Assistant Professor, SENSE
8085 and 8086 Microprocessors 21
INTERRUPT
Internal Interrupt Priority
Internally, the 8085 implements an interrupt priority scheme.
– The interrupts are ordered as follows:
TRAP
RST 7.5
RST 6.5
RST 5.5
INTR
– However, TRAP has lower priority than the HOLD signal used for DMA.
Dr. Subhasish Mahapatra
Sr. Assistant Professor, SENSE
8085 and 8086 Microprocessors 22
INTERRUPT
Dr. Subhasish Mahapatra
Sr. Assistant Professor, SENSE
8085 and 8086 Microprocessors 23
Please follow the instructions of the government and stay safe
Dr. Subhasish Mahapatra
Sr. Assistant Professor, SENSE
8085 and 8086 Microprocessors 24
