Practical 4-Module 2

Practical 4-Module 2 Demonstration experiment a. Study interrupts in 8085, RIM and SIM instructions. b. Demonstrate programs and write algorithm using RIM and SIM instructions. c. Study the interfacing of external devices with input output ports. Working with interrupts is one of the more advanced "pro-level" skills in 8085 programming. It allows the processor to handle urgent external events without constantly checking (polling) for them. a. Study of Interrupts: RIM and SIM The 8085 has five hardware interrupts: TRAP, RST 7.5, RST 6.5, RST 5.5, and INTR. • SIM (Set Interrupt Mask): Used to enable/disable (mask) specific interrupts and to send serial data out (SOD pin). • RIM (Read Interrupt Mask): Used to check the current status of interrupts (which are masked, which are pending) and to read serial data in (SID pin). The Control Word (Accumulator) for SIM: • Bits 0-2: Mask bits for RST 5.5, 6.5, and 7.5 (1 = Mask/Disable). • Bit 3 (MSE): Mask Set Enable (Must be 1 to change masks). • Bit 4 (R7.5): Reset RST 7.5 flip-flop. • Bit 6-7: Serial Output Data control. b. Program Demonstration: Using SIM and RIM This program demonstrates how to unmask (enable) the RST 7.5 interrupt and then use RIM to check if an interrupt is pending. Algorithm 1. Initialize: Load the Accumulator with the control word to unmask RST 7.5 and enable mask setting. 2. Execute SIM: Apply the control word. 3. Enable: Use the EI (Enable Interrupts) instruction to globally allow interrupts. 4. Wait: Loop or perform a task while waiting for an external signal on the RST 7.5 pin. 5. Check Status: Execute RIM to read the interrupt status into the Accumulator. 6. Analyze: Check Bit 6 of the RIM result to see if RST 7.5 is "Pending." Code in Tabular Form Label WAIT: Mnemonic Description MVI A, 08H Load 00001000 (MSE=1, Masks for 7.5/6.5/5.5 = 0) SIM Unmask RST 7.5, 6.5, and 5.5 EI Enable Interrupts globally RIM Read Interrupt Mask status into Accumulator ANI 40H Mask all bits except Bit 6 (RST 7.5 Pending bit) JZ WAIT If Bit 6 is 0, no interrupt happened; keep waiting MVI A, 01H If Bit 6 is 1, interrupt was detected STA 2500H Store "1" at 2500H to signal detection HLT Halt program c. Interfacing External Devices (I/O) Interfacing is how the 8085 communicates with the "outside world" like LEDs, Keyboards, or Sensors. There are two main methods: 1. Memory Mapped I/O: The device is treated like a memory location (uses MOV, LDA, STA). You have a huge address space ($64$ KB) but lose memory slots. 2. I/O Mapped I/O (Peripheral I/O): Uses specific instructions IN and OUT. It has its own address space ($00H$ to $FFH$, giving 256 ports). Simple I/O Interfacing Code This program reads a byte from an input switch (Port 01H) and sends it to an LED display (Port 02H). Mnemonic Description IN 01H Read 8-bit data from Input Port 01H into Accumulator CMA Complement Accumulator (often needed for common-anode LEDs) OUT 02H Send the data to Output Port 02H HLT Terminate Quick Summary Table: RIM vs SIM Feature RIM SIM Direction CPU $\leftarrow$ Interrupt System CPU $\rightarrow$ Interrupt System Main Use Check pending interrupts Mask/Unmask interrupts Serial I/O Reads SID pin Writes SOD pin Bit 3 Role Indicates IE flag status Mask Set Enable (MSE)