Download Class 7.1 Integers Characters Input Output

Class 7.1

Integers and Characters Input/Output

System Calls. Syscall

Trap Handler Services for Integers and Characters

Read Integer, Print Integer, Read Character, Print Character, Example Program

No Input Line Buffer correction.
Textbook: Appendix A (a43), System Calls
Central Connecticut State University, MIPS Tutorial. Chapter 22.
Integers and Characters Input / Output
Up until now, your programs have been running with SPIM used as a computer with no machine
code in it but your own.
Organizing Input/Output by the user program is very costly process causing duplication of
significant parts of the program code.
In even the simplest computer, putting a character on the screen involves many instructions and
a detailed knowledge of the video card.
One of the Operating System’s task is to implement all these input output operations and allow
the user programs to use the high level input/output services provided by the OS.
Most computer systems run under the control of an operating system. Application programs use
the services of the operating system to do input and output and other system tasks.
User Program
Direct I/O
System Call
Operating System
Input/Output
1
Different Operating Systems provide different I/O services for user programs.
SPIM does not have an operating system, but it can run a small trap handler — a set of services
that is a great help in writing assembly programs.
SPIM includes a "trap handler" that simulates a tiny operating system that can do input from the
keyboard and output to the monitor.
System Calls. syscall
Assembly language programs request operating system services using the
syscall instruction.
The syscall instruction transfers control to the operating system which then performs the
requested service. Then control (usually) returns to the program. (This description leaves out
many details).
syscall
# ask the operating system to perform a service
Current Program running
on the Computer
Service Call by
program itself
The Current Program is
interrupted
Current Program
Continues from the
interrupted point
Interrupt Servicing
Program in
Computer
Interrupt is
Serviced
Different operating systems use this instruction in different ways. For the SPIM trap handler
integer and character services it is used like this:
ori
$2,$0,code
# Load $2($v0) with the "code"
number of an OS service.
.......
# Put parameters for the service
in register $4 ($a0)
syscall
# Invoke the operating system.
# Return value (if any) is in $2
2
Different services use different registers, and not all services return values to the caller.
Trap Handler Services for Integers and Characters
Here are some of the services of the SPIM trap handler. The following pages explain how to use
them. The print services write characters to the simulated monitor of SPIM. The read services
read characters from the keyboard and (for numeric read services) convert character strings into
the appropriate type.
Service
Code in
$2 ($v0)
print integer
1
read integer
5
print character
11
read character
12
Arguments
Returned Value
($a0) $4 == integer to print
$2 ($v0) <-- integer
($a0) $4 == character to print
$2 ($v0) <-- character
Read Integer, Print Integer
The read integer service reads an entire line of input from your keyboard—all the characters you
type up to the newline character. These characters are expected to be ASCII digits '0', '1', .., '9'
with an optional leading '-' or '+'. The characters are converted into a 32-bit two's complement
representation of the integer which is returned in $v0.
ori
$2,$0,5
syscall
#
#
#
#
#
code 5 == read integer
Invoke the operating system.
Read in one line of ascii characters.
Convert them into a 32-bit integer.
($v0) $2 <-- two's comp. int.
The terminal gives to computer only one character at a time.
That is why to get any other than character type of information (integer, string) OS reads
characters one by one and translates them to the type we need.
$a0 to the SPIM
terminal. Of course, there are many ways that the integer can be placed in $a0.
The print integer service prints the integer represented by the 32 bits in
ori
$2,$0,1
# code 1 == print integer
ori
$4,$0,126 # $a0 == the integer
syscall
# Invoke the operating system.
# Convert the 32-bit integer into characters.
# Print the character to the monitor.
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Read Character, Print Character
The read character service reads one character from your keyboard. The character’s ASCII code
is returned in $v0.
The print character service prints the character represented in the lowest order byte of the 32
bits in $a0 to the SPIM terminal.
Characters 0x0D and 0x0A could be used for moving the console cursor to the next line’s first
position. This is the standard End Of Line “\n” character’s behavior.
Example Program
Below is the example program code with both integer/character input/output.
## integer/character input/output.asm
##
.text
.globl
__start
__start:
ori
ori
syscall
$4,$0,129
$2,$0,1
# prepare integer in $4 ($a0)
# print integer from $4
ori
ori
syscall
$4,$0,10
$2,$0,11
# prepare character EOL in $4 ($a0)
# print character End Of line
ori
syscall
$2,$0,5
# read integer to $2($v0)
or
ori
syscall
$4,$0,$2
$2,$0,1
# print integer after reading
ori
ori
syscall
$4,$0,13
$2,$0,11
# prepare character EOL in $4 ($a0)
# print character End Of line
ori
syscall
$2,$0,12
# read character to $2($v0)
or
ori
syscall
## End of file
$4,$0,$2
$2,$0,11
# copy read character from $2 to $4
# print the character which was read
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No Input Line Buffer Correction
The user might make a mistake in entering the character and try to correct it by hitting the
"backspace" key to erase the mistake. But this does not work. The ascii value of the key 0x08 is
included in the string just as any character value. Odd byte values show up (in SPIM) as a small
black rectangle.
Most operating system user interfaces allow the user to edit a string (with BS, DEL, arrow and
other keys) before it is sent to an application program. The application program asks for a string
from the user, but the OS does the actual input, complete with user editing. (This is sometimes
called "cooked" input mode).
The SPIM trap handler service does not do this. Characters are sent directly to the program as
they were typed. If the user hits the backspace character, then that character is placed in the
input string. (This is called "raw" input mode).
You could, of course, write your program so that it looks at the raw characters in the buffer and
edits the input string. This is a messy task, and often skipped, but should not be.
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