1. Interrupts

2. Learning Objectives I Describe the interrupt process
Explain the difference between a nonmaskable and a maskable interrupt
Explain the difference between a direct and a vectored interrupt
List the priority of interrupt in 8085
List the 8085 vectored interrupts, nonmaskable interrupt and their vectored addresses

3. What is an Interrupt ? Hardware interrupt is a CPU facility
- which permits spurious asynchronous events to suspend program execution
- and instead execute a software module to service the event
Interrupt pin :-
- connection to the processor which allows external devices to signal a request for service
Interrupt service routine ( ISR ) :-
- software module that the processor executes in response to an interrupt
Interrupt mechanism :-
- After completion of the ISR , the processor returns to execution of the main program from the point at which it was interrupted

4. Interrupt Event Sequence1
normal
processing 2
interrupt
occurs 7
resume normal
processing 3
save PC
on stack 6
retrieve PC
from stack 4
branch to
ISR 5
execute
ISR

5. Maskable and Non-maskable Interrupts
if asserted, will always interrupt the processor
no software mechanism to prevent the processor being interrupted by a non-maskable interrupt
Maskable interrupt :-
if asserted, will only interrupt the processor if it is enabled (unmasked)
Maskable interrupts can be enabled (unmasked) or disabled (masked) by software
Most maskable interrupts automatically become disabled (masked) after an interrupt has occurred. It requires further software commands to re-enable maskable interrupts.

6. Direct and Vectored Interrupts
Direct interrupts :-
interrupting device need to provide the interrupt signal only (i.e. to assert the signal to the interrupt pin of the processor)
the address of the first instruction of the ISR for the particular interrupt is pre-programmed into the CPU
Vectored interrupts :-
- interrupting device has to supply both the interrupt signal and the 16-bit address of the first instruction of the ISR.
- Interrupt service routines can reside anywhere in the memory map of the computer system.

7. Interrupt Priority
For processor with multiple interrupt input pins, the various interrupts are assigned a priority.
When simultaneous interrupts occur the highest priority interrupt will be serviced before lower priority interrupts
It is possible to arrange software such that whilst a lower priority interrupt is being serviced that a higher priority interrupt can interrupt the lower priority service routine
1/12/2019

8. 8085A Interrupts 8085A TRAP RST7.5 RST6.5 RST 5.5 INTR INTA
8085A has 5 interrupts inputs :-
(a) TRAP
- Only non-maskable interrupt
- It is direct interrupt
(b) RST 7.5, RST 6.5, RST 5.5
- Direct interrupt and maskable interruprt
(c) INTR
- Only vector interrupt
- Maskable interruprt
8085A
TRAP
RST7.5
RST6.5
RST 5.5
INTR
INTA
* INTA - Interrupt acknowledge signal

9. 8085A Interrupts
Interrupt pins at 6, 7, 8, 9 and 10 for TRAP, RST 7.5, RST 6.5 and
INTR respectively

10. 8085A Interrupts

11. Rising Edge AND High Level
Interrupt
Trigger Type
TRAP
Rising Edge AND High Level
RST 7.5
Rising Edge
RST 6.5
High Level
RST 5.5
INTR
Notes : Rising Edge - Positive edge-triggered interrupt
High Level - Level-triggered interrupt

12. Learning Objectives II
List the instructions related to interrupt process
Explain the function of these instructions in the 8085 interrupt process
Describe the process of enabling and disabling
- the maskable, direct and vectored interrupts

13. Enabling and Disabling Maskable Interrupts
Maskable interrupts - RST 7.5, RST 6.5, RST 6.5, INTR
- only interrupt the processor if it is enabled (unmasked)
- Maskable interrupts can be enabled (unmasked) or disabled (masked) by software
Instruction EI (Enable Interrupt)
- 1-byte instruction
- set the Interrupt Enable flip-flop and enables the interrupts process
- system reset or an interrupt disables after the interrupt process is done
Instruction DI (Disable Interrupt)
- 1-byte instruction
- reset the Interrupt Enable flip-flop and disables the interrupts process
The individual mask for RST 5.5, RST 6.5 and RST 7.5 are manipulated using SIM instruction
- this instruction takes the bit pattern in the Accumulator and applies it to the interrupt mask enabling and disabling the specifies interrupts

14. How SIM Interprets the Accumulator
SDO
SDE
XXX
R7.5
MSE
M7.5
M6.5
M5.5 1 2 3 4 5 6 7
Enable Serial Data
0 - Ignore bit 7
1 - Send bit 7 to SOD pin
Serial Data Out
Not Used
RESET RST7.5 :
If =1  RST 7.5 Flip Flop to reset OFF
Mask Set Enable
0 - Ignore bits 0-2
1 - Set the masks according
to bits 0-2
RST5.5 Mask
RST6.5 Mask
RST7.5 Mask }
0 – Available
1 - Masked

15. Bit 3 (Mask Set Enable - MSE) is an enable for setting the mask.
Bit 0 is the mask for RST 5.5, bit 1 is the mask for RST 6.5 and bit 2 is the mask for RST 7.5.
If the mask bit is 0, the interrupt is available.
If the mask bit is 1, the interrupt is masked.
Bit 3 (Mask Set Enable - MSE) is an enable for setting the mask.
If it is set to 0 the mask is ignored and the old settings remain.
If it is set to 1, the new setting are applied.
The SIM instruction is used for multiple purposes and not only for setting interrupt masks.
It is also used to control functionality such as Serial Data Transmission.
Therefore, bit 3 is necessary to tell the microprocessor whether or not the interrupt masks should be modified

16. Using SIM instruction to Modify the Interrupt Mask
Example 1 : Set the interrupt masks so that RST 7.5 and RST 5.5
is enable and disable RST6.5
Determine the contents of the Accumulator
Enable RST bit 0 = 0
Disable RST bit 1 = 1
SDO
SDE
XXX
R7.5
MSE
M7.5
M6.5
M5.5
Enable RST bit 2 = 0
Allow setting the masks bit 3 = 1 1 1
Don’t reset the flip flop bit 4 = 0
Bit 5 is not used bit 5 = 0
Don’t used serial data bit 6 = 0
Contents of accumulator are: 0AH
Serial data is ignored bit 7 = 0
MVI A, 0A H
SIM ;Set Interrupt Mask
EI ;Enable Interrupt

17. Using SIM instruction to Modify the Interrupt Mask
Example 2 : Reset RST 7.5 interrupt from example 1
Determine the contents of the Accumulator
Enable RST bit 0 = 0
Disable RST bit 1 = 1
SDO
SDE
XXX
R7.5
MSE
M7.5
M6.5
M5.5
Enable RST bit 2 = 0
Allow setting the masks bit 3 = 1 1 1 1
Reset the flip flop bit 4 = 1
Bit 5 is not used bit 5 = 0
Don’t used serial data bit 6 = 0
Contents of accumulator are: 1AH
Serial data is ignored bit 7 = 0
MVI A, 1A H
SIM ;Set Interrupt Mask
EI ;Enable Interrupt

18. Reading The Status of Maskable Interrupts
RIM instruction :-
- 1 byte instruction
- To read the status of maskable interrupts. This instruction loads the accumulator with 8bits indicating the current status of the interrupt masks
- to identify pending interrupts – Bit D4, D5 and D6
- to receive serial data – Bit D7
SDI
P7.5
P6.5
P5.5 IE
M7.5
M6.5
M5.5 1 2 3 4 5 6 7
RST5.5 Interrupt Pending
RST6.5 Interrupt Pending
RST7.5 Interrupt Pending }
1 - Pending
Serial Data In
Interrupt Enable
Value of the Interrupt Enable
Flip Flop [1 – Enable]
RST5.5 Mask
RST6.5 Mask
RST7.5 Mask }
0 - Available
1 - Masked

19. Direct Interrupts [ TRAP, RST 5.5, RST 6.5, RST 7.5 ]
BI-bus idle, MW- memory Write

20. Direct Interrupt Process
The interrupt process should be enabled using the EI instruction.
The 8085 checks for an interrupt during the execution of every instruction.
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.
The microprocessor then executes a call instruction that sends the execution to the appropriate location in the interrupt vector table.
When the microprocessor executes the call instruction, it saves the address of the next instruction on the stack.
The microprocessor jumps to the specific service routine.
The service routine must include the instruction EI to re-enable the interrupt process.
At the end of the service routine, the RET instruction returns the execution to where the program was interrupted.
BI-bus idle, MW- memory Write

21. Machine Cycles with Direct Interrupts
Since there is no requirement to supply ISR addresses with direct interrupts ( TRAP, RST 5.5, RST 6.5 & RST 7.5 ) then there is no requirement for the 8085A to execute INTA machine cycles in response to such interrupts.
However to provide the CPU sufficient time to process direct interrupt - six T-state bus idle machine cycle is introduced, following recognition of the direct interrupt.
During the bus idle machine cycle, no control signal is asserted nor is the program counter incremented. Ready line control is ignored during the bus idle cycle.
Following with the bus idle cycle which is two memory write cycles are executed to save the current contents of the program counter on the stack.
The program counter is then overwritten with the pre-programmed address for the particular interrupt source.

22. Machine Cycles with Direct Interrupts
Normal operation cycles
BI-bus idle, MW- memory Write
* No INTA machine cycle – Direct Interrupt
Since there is no requirement to supply ISR addresses with direct interrupts ( TRAP, RST 5.5, RST 6.5 & RST 7.5 ) then there is no requirement for the 8085A to execute INTA machine cycles in response to such interrupts.

23. Machine Cycles with Direct Interrupts
BI-bus idle, MW- memory Write
* No INTA machine cycle – Direct Interrupt
However to provide the CPU sufficient time to process direct interrupt - six T-state bus idle machine cycle is introduced, following recognition of the direct interrupt.

24. Machine Cycles with Direct Interrupts
BI-bus idle, MW- memory Write
* No INTA machine cycle – Direct Interrupt
During the bus idle machine cycle, no control signal is asserted nor is the program counter incremented. Ready line control is ignored during the bus idle cycle.

25. Machine Cycles with Direct Interrupts
BI-bus idle, MW- memory Write
* No INTA machine cycle – Direct Interrupt
Following with the bus idle cycle which is two memory write cycles are executed to save the current contents of the program counter on the stack.

26. Machine Cycles with Direct Interrupts
BI-bus idle, MW- memory Write
* No INTA machine cycle – Direct Interrupt
The program counter is then overwritten with the pre-programmed address for the particular interrupt source.

27. Vector Interrupts [ INTR ]
BI-bus idle, MW- memory Write

28. Vector Interrupts
INTR is a vector interrupt and it is also maskable interrupt
Interrupt process - enabled using EI instruction.
RST instruction – used to transfer the program control to specific memory location
The Restart instruction, RST n, where 0 ≤ n ≤ 7
RST n
((SP)-1)  (PCH)
((SP)-2)  (PCL)
((SP)  (SP) – 2
(PC)  8 * n

29. RST n
In respond to the INTA strobe, external logic places an RST n opcode on the data bus.
RST n has the following bit pattern
11NNN111
where n=NNN (3 bit binary number) and restart address is n * 8
For example if :-
- RST 1, NNN = 001 and restart address is 08H (001*8)
- RST 2 (address 10H), RST 3 (address 18H) and etc.

30. RST n Mnemonic Binary Code Hex Code Call Location in Hex D7 D6 D5 D41 C7
0000
RST 1 CF
0008
RST 2
0010
RST 3 DF
0018
RST 4 E7
0020
RST 5 EF
0028
RST 6 F7
0030
RST 7 FF
0038

31. Vector Interrupt Process
The interrupt process should be enabled using the EI instruction.
The 8085 checks for an interrupt during the execution of every instruction.
If INTR is high, MP completes current instruction, disables the interrupt and sends INTA (Interrupt acknowledge) signal to the device that interrupted
INTA allows the I/O device to send a RST instruction through data bus.
Upon receiving the INTA signal, MP saves the memory location of the next instruction on the stack and the program is transferred to ‘call’ location (ISR Call) specified by the RST instruction
Microprocessor Performs the ISR.
ISR must include the ‘EI’ instruction to enable the further interrupt within the program.
RET instruction at the end of the ISR allows the MP to retrieve the return address from the stack and the program is transferred back to where the program was interrupted.
BI-bus idle, MW- memory Write

32. Machine Cycles with Vectored Interrupts
The 8085A processor executes a number of machine cycles, in response to a vectored interrupt (INTR), prior to execution of the first instruction of the interrupt service routine.
The processor completes the execution of the current instruction. (Note : The processor only samples the interrupt inputs in the last T-state of the last machine cycle in the current instruction cycle)
This has implication in system design as it means that the interrupt signal on INTR must remain in the asserted state for at least the longest instruction in the 8085A instruction set to guarantee that the processor recognizes the interrupt.

33. Machine Cycles with Vectored Interrupts
The processor then executes a six T-state interrupt acknowledge machine cycle ( the INTA machine cycle is similar to the opcode fetch machine cycle except that the program counter is not incremented and the INTA* control signal is asserted instead of RD*)
In response to the INTA* signal, the interrupting device need to place the opcode of an instruction onto the data bus ( called jamming ).
The processor reads the opcode in the normal manner and stores it in the instruction register.
The choice of opcode is restricted as it is necessary to automatically save the contents of the program counter to enable the program to return to the point in the software where it was interrupted.

34. Machine Cycles with Vectored Interrupts (CALL)
The only viable choice of 8085A instruction is either the CALL instruction or the RST n instruction.
The CALL instruction is a 3-byte instruction with bytes 2 & 3 being the address of the first instruction of the subroutine ( in this case the interrupt service routine).
Following decoding of the call opcode, the processor executes a further two interrupt acknowledge machine cycles to fetch the address of the start of the ISR.
It is incumbent on the interrupting device to place the low byte of the address of the ISR onto the data bus in response to the second INTA* control signal and the high byte of the address in response to the third INTA* signal.

35. Machine Cycles with Vectored Interrupts (CALL)
The execution phase of the CALL instruction can now take place.
The processor firstly executes two memory write machine cycles to save the current contents of the program counter onto the stack. The address as to where in memory the contents of PC is to be saved is specified by the stack pointer register.
Finally the processor overwrites the contents of the program counter with the second and third bytes of the call instruction.
The next instruction the processor will execute will be the first instruction of the ISR.

36. Machine Cycles with Vectored Interrupts
The processor then executes a six T-state interrupt acknowledge machine cycle ( the INTA machine cycle is similar to the opcode fetch machine cycle except that the program counter is not incremented and the INTA* control signal is asserted instead of RD*)

37. Machine Cycles with Vectored Interrupts
In response to the INTA* signal, the interrupting device need to place the opcode of an instruction onto the data bus ( called jamming ).
The processor reads the opcode in the normal manner and stores it in the instruction register.

38. Machine Cycles with Vectored Interrupts
The choice of opcode is restricted as it is necessary to automatically save the contents of the program counter to enable the program to return to the point in the software where it was interrupted.

39. Machine Cycles with Vectored Interrupts
The only viable choice of 8085A instruction is either the CALL instruction or the RST n instruction.
The CALL instruction is a 3-byte instruction with bytes 2 & 3 being the address of the first instruction of the subroutine ( in this case the interrupt service routine).

40. Machine Cycles with Vectored Interrupts
Following decoding of the call opcode, the processor executes a further two interrupt acknowledge machine cycles to fetch the address of the start of the ISR.

41. Machine Cycles with Vectored Interrupts
Low Byte in 2nd INTA
It is incumbent on the interrupting device to place the low byte of the address of the ISR onto the data bus in response to the second INTA* control signal and the high byte of the address in response to the third INTA* signal.

42. Machine Cycles with Vectored Interrupts
High Byte in 3rd INTA
It is incumbent on the interrupting device to place the low byte of the address of the ISR onto the data bus in response to the second INTA* control signal and the high byte of the address in response to the third INTA* signal.
The execution phase of the CALL instruction can now take place.

43. Machine Cycles with Vectored Interrupts
The execution phase of the CALL instruction can now take place.
The processor firstly executes two memory write machine cycles to save the current contents of the program counter onto the stack. The address as to where in memory the contents of PC is to be saved is specified by the stack pointer register.

44. Machine Cycles with Vectored Interrupts
Finally the processor overwrites the contents of the program counter with the second and third bytes of the call instruction.
The next instruction the processor will execute will be the first instruction of the ISR.