MARS MIPS - A Program to echo output from the Keyboard

Introduction

The aim to write a program that demonstrates the use of peripheral devices like keyboard. The program should echo what is being getting typed in the keyboard. The post describes the code that generates the output shown in the video.

This is me practicing basic stuff to be done in the MIPS assembly language program.

Code Section

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# Module 10 Memory Mapped I/O Demo # Memory mapped address of device registers. # 0xFFFF0000 rcv contrl # 0xFFFF0004 rcv data # 0xFFFF0008 tx contrl # 0xFFFF000c tx data .data .eqv MMIOBASE 0xffff0000 # Receiver Control Register (Ready Bit) .eqv RCR_ 0x0000 .eqv RCR RCR_($s0) # Receiver Data Register (Key Pressed - ASCII) .eqv RDR_ 0x0004 .eqv RDR RDR_($s0) # Transmitter Control Register (Ready Bit) .eqv TCR_ 0x0008 .eqv TCR TCR_($s0) # Transmitter Data Register (Key Displayed- ASCII) .eqv TDR_ 0x000c .eqv TDR TDR_($s0) .text .globl main main: li $s0,MMIOBASE # get base address of MMIO area keyWait: lw $t0,RCR # get control reg andi $t0,$t0,1 # isolate ready bit beq $t0,$zero,keyWait # is key available? if no, loop lbu $a0,RDR # get key value displayWait: lw $t1,TCR # get control reg andi $t1,$t1,1 # isolate ready bit beq $t1,$zero,displayWait # is display ready? if no, loop sb $a0,TDR # send key to display li $v0,11 syscall j keyWait

Here's a high-level overview of what the code is doing:

1. It defines memory-mapped addresses for four registers: Receiver Control Register (RCR), Receiver Data Register (RDR), Transmitter Control Register (TCR), and Transmitter Data Register (TDR).

2. In the main function, it initializes a register $s0 with the MMIO base address.

3. It enters a loop that waits for a keypress by checking the value of the ready bit in the RCR register. Once a keypress is detected, the code retrieves the ASCII code of the key from the RDR register.

4. The code then enters another loop that waits for the display to be ready to receive data by checking the value of the ready bit in the TCR register. Once the display is ready, the code sends the ASCII code of the key to the display by storing it in the TDR register.

5. Finally, the code calls the syscall function to display the key on the screen and then jumps back to the beginning of the loop to wait for the next keypress.

Conclusion

This assembly language program that demonstrates Memory-Mapped I/O (MMIO). MMIO is a method of communication between the CPU and peripheral devices where memory locations are used to represent the device's control and data registers.