Set Interrupt Mask (SIM) instruction

Set Interrupt Mask (SIM) instruction

Set Interrupt Mask (SIM) – It is used to implement the hardware interrupts (RST 7.5, RST 6.5, RST 5.5) by setting various bits to form masks or generate output data via the Serial Output Data (SOD) line. First the required value is loaded in accumulator then SIM will take the bit pattern from it.

Instruction SIM: Set Interrupt Mask. This is a 1-byte instruction and can be used for three different functions.

• one function is set mask for RST 7.5, RST 6.5 and RST 5.5 interrupts. This instruction reads the content of the accumulator and enables or disables the interrupts according to the content of the accumulator. Bit D3 is a control bit and should =1 for bit D0, D1 and D2 to be effective. Logic 0 on D0, D1, and D2 will enable the corresponding interrupts, and logic 1 will disable the interrupts.
• The second function is to reset RST 7.5 flip-flop. Bit D4 is additional control for RST 7.5. If D4=1, RST 7.5 reset. This is used to override (or ignore) RST 7.5 without servicing it.
• The third function is to implement serial I/O. Bit D7 and D6 of the accumulator are used for serial I/O and do not affect the interrupts. Bit D6=1 enable the I/O and bit D7 is used to transmit (output) bits. 

Figure :- Interpretation of the Accumulator Bit Pattern for the SIM Instruction

Table :- Interpretation of the Accumulator Bit Pattern for the SIM Instruction

Pin Number	Pin Name	Pin Details	Remark
D0	M 5.5	RST 5.5 MASK	0= Available/ Unmasked 1= Masked
D1	M 6.5	RST 6.5 MASK
D2	M 7.5	RST 7.5 MASK
D3	MSE	Mask Set Enable	If 0= bit 0-2 ignored If 1= Mask is set according to bit 0-2
D4	RST 7.5	Reset	RST 7.5: If 1, RST 7.5 flip-flop is reset OFF
D5	XXX	Ignored/ Not used
D6	SDE	Serial Data Enable	If 0= Ignore Bit 7 If 1=bit 7 is output to serial output data latch.
D7	SOD	Serial Output Data	Ignore if bit D6=0

REFERENCES

1. R. S. Gaonkar, Microprocessor Architecture, Programming, and Applications with the 8085, Fifth Edition, Penram International Publishing (India) Private Limited.
2. S Ghoshal, Microprocessor Based System Design, Macmillan India Limited, 1996
3. M. Mano, Digital Logic and Computer Design, Prentice – Hall India
4. B. Ram - Fundamentals of Microprocessor and Microcontrollers
5. “Microprocessors: Principles and Applications” by A Pal
6. “Microprocessors and Microcontrollers : Architecture, Programming and Interfacing Using 8085, 8086 and 8051” by Soumitra Kumar Mandal
7. “Introduction to Microprocessors and Microcontrollers” by Crisp John Crisp
8. “Microprocessors And Microcontrollers” by A Nagoor Kani
9. “Microprocessors And Microcontrollers : Architecture, Programming and System Design 8085, 8086, 8051, 8096” by Krishna Kant
10. “8 - Bit Microprocessor” by Vibhute