Interrupt is the mechanism by which the processor is made to transfer control from its current program execution to another program having higher priority. The interrupt signal may be given to the processor by any external peripheral device.
The program or the routine that is executed upon interrupt is called interrupt service routine (ISR). After execution of ISR, the processor must return to the interrupted program. Key features in the interrupt structure of any microprocessor are as follows:
i. Number and types of interrupt signals available.
ii. The address of the memory where the ISR is located for a particular interrupt signal. This address is called interrupt vector address (IVA).
iii. Masking and unmasking feature of the interrupt signals.
iv. Priority among the interrupts.
v. Timing of the interrupt signals.
vi. Handling and storing of information about the interrupt program (status information).
Types of Interrupts:
Interrupts are classified based on their maskability, IVA and source. They are classified as:
i. Vectored and Non-Vectored Interrupts
· Vectored interrupts require the IVA to be supplied by the external device that gives the interrupt signal. This technique is vectoring, is implemented in number of ways.
· Non-vectored interrupts have fixed IVA for ISRs of different interrupt signals.
ii. Maskable and Non-Maskable Interrupts
· Maskable interrupts are interrupts that can be blocked. Masking can be done by software or hardware means.
· Non-maskable interrupts are interrupts that are always recognized; the corresponding ISRs are executed.
iii. Software and Hardware Interrupts
· Software interrupts are special instructions, after execution transfer the control to predefined ISR.
· Hardware interrupts are signals given to the processor, for recognition as an interrupt and execution of the corresponding ISR.
Interrupt Handling Procedure:
The following sequence of operations takes place when an interrupt signal is recognized:
i. Save the PC content and information about current state (flags, registers etc) in the stack.
ii. Load PC with the beginning address of an ISR and start to execute it.
iii. Finish ISR when the return instruction is executed.
iv. Return to the point in the interrupted program where execution was interrupted.
Interrupt Sources and Vector Addresses in 8085:
8085 instruction set includes eight software interrupt instructions called Restart (RST) instructions. These are one byte instructions that make the processor execute a subroutine at predefined locations. Instructions and their vector addresses are given in Table 6.
Table 6 Software interrupts and their vector addresses
The software interrupts can be treated as CALL instructions with default call locations. The concept of priority does not apply to software interrupts as they are inserted into the program as instructions by the programmer and executed by the processor when the respective program lines are read.
Hardware Interrupts and Priorities:
8085 have five hardware interrupts – INTR, RST 5.5, RST 6.5, RST 7.5 and TRAP. Their IVA and priorities are given in Table 7.
Table 7 Hardware interrupts of 8085
Masking of Interrupts:
Masking can be done for four hardware interrupts INTR, RST 5.5, RST 6.5, and RST 7.5. The masking of 8085 interrupts is done at different levels. Fig. 13 shows the organization of hardware interrupts in the 8085.
The Fig. 13 is explained by the following five points:
i. The maskable interrupts are by default masked by the Reset signal. So no interrupt is recognized by the hardware reset.
ii. The interrupts can be enabled by the EI instruction.
iii. The three RST interrupts can be selectively masked by loading the appropriate word in the accumulator and executing SIM instruction. This is called software masking.
iv. All maskable interrupts are disabled whenever an interrupt is recognized.
v. All maskable interrupts can be disabled by executing the DI instruction.
RST 7.5 alone has a flip-flop to recognize edge transition. The DI instruction reset interrupt enable flip-flop in the processor and the interrupts are disabled. To enable interrupts, EI instruction has to be executed.
The SIM instruction is used to mask or unmask RST hardware interrupts. When executed, the SIM instruction reads the content of accumulator and accordingly mask or unmask the interrupts. The format of control word to be stored in the accumulator before executing SIM instruction is as shown in Fig. 14.
In addition to masking interrupts, SIM instruction can be used to send serial data on the SOD line of the processor. The data to be send is placed in the MSB bit of the accumulator and the serial data output is enabled by making D6 bit to 1.
RIM instruction is used to read the status of the interrupt mask bits. When RIM instruction is executed, the accumulator is loaded with the current status of the interrupt masks and the pending interrupts. The format and the meaning of the data stored in the accumulator after execution of RIM instruction is shown in Fig. 15.
In addition RIM instruction is also used to read the serial data on the SID pin of the processor. The data on the SID pin is stored in the MSB of the accumulator after the execution of the RIM instruction.
Ex: Write an assembly language program to enables all the interrupts in 8085 after reset.
EI : Enable interrupts
MVI A, 08H : Unmask the interrupts
SIM : Set the mask and unmask using SIM instruction
Timing of Interrupts:
The interrupts are sensed by the processor one cycle before the end of execution of each instruction. An interrupts signal must be applied long enough for it to be recognized. The longest instruction of the 8085 takes 18 clock periods. So, the interrupt signal must be applied for at least 17.5 clock periods. This decides the minimum pulse width for the interrupt signal.
The maximum pulse width for the interrupt signal is decided by the condition that the interrupt signal must not be recognized once again. This is under the control of the programmer.