1. Ken D. Nguyen Department of Computer Science Georgia State University
I/O: SPARC Assembly
Ken D. Nguyen
Department of Computer Science
Georgia State University

2. I/O on SPARC (File I/O will be covered in chapter 10)
I/O thru buffers allocated for each terminal is an tremendous work in assembly.
In SPARC, using system call printf and scanf is an easy way to perform I/O
These functions perform conversion and extraction automatically.
Your data could be resided in any memory block (.data, .bss, or stack – for stack)

3. printf
To use printf function, at least the following must be satisfied:
%o0 must be containing the address of a null terminated string (c-string). A null terminated string is a string ended with 1 byte zero.
Ex:
!in your data section
myString: .asciz “hello world!”
. . .
!and in your code
set myString, %o0 ! Get the address of the label
call printf !this should print hello world!
nop

4. printf cont.
printf allows you to substitute some other data into your output.
If your string pointed by %o0 contains %F, you must have your register %o1-%o5 containing the values to be substituted in the same order as the %F (except string), where F is one of the following
d or i Decimal signed integer.
o Octal integer.
x or X Hex integer.
U Unsigned integer.
c Character.
s C-String, i.e. null terminated string.
f Double
e or E Double.
g or G Double.
n Number of characters written by this printf.

5. Output formats o 0 prefix inserted.
x or X (Hex) 0x prefix added to non-zero values.
e or E Always show the decimal point.
f Always show the decimal point.
g or G Always show the decimal point trailing zeros not removed.

6. printf examples !assuming your string declared as follow
myString: .asciz “it’s %d dollars” …
!In your code
set myString, %o0 ! The address of the string
call printf !will print it’s 20 dollars
mov 20, %o1 !substitutes for the first %d

7. printf examples cont. !assuming your string declared as follow
myString: .asciz “it’s %d dollars and %d cents” …
!In your code
set myString, %o0 ! The address of the string
mov 20, %o1 ! substituted for the first %d
call printf ! Will print it’s 20 dollars and 5 cents
mov 5, %o2 ! substituted for the second %d

8. Printing a string
To use printf to print a string, the address of the substituted string must be in the correspondent register.
Ex:
!assuming your strings are in data section as follows
mystring: .asciz “Hello %s!”
name: .asciz “Michael”
. . .
! and your code should be
set myString, %o0 ! address of the output
set name, %o1 ! Address of the substituted string
call printf ! Will print Hello Michael!
nop

9. scanf scanf is used for reading user input
scanf requires %o0 containing the address of a c-string containing the input format
Subsequence registers (%o1-%o5) contain the address of the memory where the input should be stored
%d and %i always give you a 32-bit (word-size) input, so the address must be aligned by word boundary (aligned by 4)

10. scanf examples !assuming your strings are in data section as follows
input: .word 0 !aligned by 4
format: .asciz “%d”
. . .
! and your code should be
set format, %o0 ! Address of the format
set input, %o1 ! Address of the location for the input
call scanf ! Reads user input, converts to a
nop ! number and stores at the memory ! referenced by input
Note: the new line (the ENTER key) the user enters will still be in the buffer, one way to extract that is to read an additional character (or flush it)

11. scanf examples cont.
!assuming your strings are in data section as follows
input: .word 0 !aligned by 4
nl: .byte 0 ! A byte to store the input \n
format: .asciz “%d%c”
. . .
! and your code should be
set format, %o0 ! Address of the format
set input, %o1 ! Address of the location for the input
set nl , %o2 ! location for a character
call scanf ! Reads user input, converts to a
nop ! number and stores at the memory ! referenced by input
This example removes the ENTER key from the buffer

12. Scanf: string example
!assuming your strings are in data section as follows
format: .asciz “%s”
input: .asciz “ ” !Allocate some space for the input string
. . .
! and your code should be
set format, %o0 ! Address of the format
set input, %o1 ! Address of the location for the input
call scanf ! Reads user input up to a blank or ENTER key
nop
Note: if the user input is longer than the space you allocated, the input will overflow to the subsequence bytes  A very well known buffer overflow problem

13. An sample program .section ".data“ /* These are variables */
prompt: .asciz "\nPlease enter your name: “
format: .asciz "%s"
format2: .asciz "Your name is:%s\n"
input: .asciz “ "
/* Program starts */
.align 4
.section ".text"
.global main
main:
save %sp, -96, %sp ! save the stack
set prompt, %o0 ! point o0 to the prompt
call printf ! call printf to print the prompt
nop
set format, %o0 ! point o0 to the input format string
set input, %o1 ! point o1 at the input variable
call scanf ! get the input into this variable
set format2, %o0 ! point o0 to the output format
set input, %o1 ! point o1 to the string to be displayed
call printf ! print the string pointed by o1
ret ! return
restore ! get out
An sample program

14. More example This program will keep re-prompting for a new input

15. Accessing your data in memory
Covering in chapter 5