The document discusses pending interrupts in the 8085 microprocessor. It explains that the 8085 has multiple interrupt lines and when one interrupt is served, others may occur and remain pending. It introduces the Read Interrupt Mask (RIM) instruction, which loads the accumulator with an 8-bit value indicating the status of interrupt masks and can identify pending interrupts. It provides details on how to interpret the accumulator bits after a RIM instruction to check for pending interrupts like RST 6.5. It includes sample assembly code to check if RST 6.5 is pending after an RST 7.5 interrupt and enable it if so before returning to the main program.

1. MICROPROCESSOR 8085
LECTURE 25
PENDING INTERRUPTS
PROF. SANDIP DAS

2. • In 8085, there are several interrupt lines.
• When one interrupt request is served, other may occur and can
remain pending.
• To sense the pending interrupts an additional instruction is there in
8085, known as Read Interrupt Mask (RIM).
RIM: It is a 1byte instruction and can be used for following functions-
• To read interrupt masks. This function loads the accumulator with 8 bits
indicating the current status of the interrupt masks.
• To identify pending interrupts.
• To receive serial data.
PENDING INTERRUPTS

3. RIM
Interpretation of the Accumulator bit pattern for the RIM
instruction
Bit 7 Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 Bit 0
SID I7.5 I6.5 I5.5 IE M 7.5 M 6.5 M 5.5
RST 7.5
MASK
RST 6.5
MASK
RST 5.5
MASK
0=
available
1= masked
Interrupt
Enable flag,
1=enabled
Pending
interrupt;
1=Pending
Serial
Input
data

4. ASSUME 8085 IS COMPLETING AN RST7.5 INTERRUPT REQUEST, CHECK TO SEE
IF RST 6.5 IS PENDING. IF IT IS PENDING ENABLE RST 6.5, OTHERWISE RETURN
TO MAIN PROGRAM
Labels Mnemonics, operandsComments
RIM Read Interrupt Mask
MOV B,A Save Mask Information
ANI 20H Check whether RST 6.5 is pending
or not
JNZ NEXT
EI
RET RST 6.5 is not pending, return to main
program
NEXT MOV A,B Get bit pattern, RST 6.5 is pending
ANI 0DH Enables RST 6.5 by setting D1 to 0
ORI 08H Enable SIM by setting D3=1
SIM