Doing I/O using ARC Memory Mapped I/O

ARC does not have any explicit I/O instructions. I/O is then done by reading from and

writing to pre-defined memory locations in the memory which are assigned to I/O

devices. This is known as memory mapped I/O. Each I/O device has a data port.

Similarly, each device has a status port that is used to test the readiness status of the

I/O device.

using the ARC simulator to use:

- The console output port to display characters

- The keyboard input port to read characters

1) Please provide comments for all instructions.

2) What is the memory mapped address for the console output and keyboard input?

3) Please identify the specific line of code that prints to console and reads the

keystroke.

PART A Output: Printing character to the display

The memory addresses associated with the console output I/O device are:

- 0xffff0000 is the console (output) data port

- 0xffff0004 is the console (output) status port where bit 7 is the ready flag (0: not

ready, 1: ready)

Printing a character is achieved by:

- Checking the ready flag to see if device is ready for printing, i.e., check if the bit 7

of the output status port is set to 1

- Storing the character to the output data port

! Prints "Hello, world! " in the message area.

.begin

BASE .equ 0x3fffc0 !Starting point of the memory mapped region

COUT .equ 0x0 !0xffff0000 Console Data Port

COSTAT .equ 0x4 !0xffff0004 Console Status Port

.org 2048

add %r0, %r0, %r2

add %r0, %r0, %r4

sethi BASE, %r4

Loop: ld [%r2 + String], %r3 !Load next char into r3

addcc %r3,%r0,%r3

be End ! stop if null

Wait: ldub [%r4+COSTAT], %r1

andcc %r1, 0x80, %r1

be Wait

stb %r3, [%r4+COUT] !Print to console

add %r2, 4, %r2 !increment String offset (r2)

ba Loop

End: halt !A non-standard instruction to stop the simulator

.org 3000

! The "Hellow, world!" string

String: 0x48, 0x65, 0x6c, 0x6c, 0x6f

0x2c, 0x20, 0x77, 0x6f, 0x72

0x6c, 0x64, 0x21, 0x0a, 0

.end