CS 220 Computer Organization

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CS 220Computer Organization

• Chapter 3 SASM (cont.)

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CS 220 Computer Organization SASM (cont.)

• SASM A simple Abstract Language
• SASM Control Structures
• conditional and iterative
• depend on condition codes for most control
  decisions
• ZF Zero Flag 1 if value is zero
• SF Sign Flag 1 if value is negative
• condition codes are set by arithmetic operations
  or the compare instruction
• Example (conditional)

compareb username, wrightj bez
valid_user moveb mess, invalid user
br end_if valid_user moveb mess,
successful login end_if
If (username .ne. wrightj) then mess
invalid user else mess successful
login end if
Fortran
SASM
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CS 220 Computer Organization SASM (cont.)

• SASM A simple Abstract Language
• SASM Control Structures
• Example (iterative)

get_username compareb username, wrightj
bez valid_user iadd errorcnt, 1 br
get_username valid_user
do while (username .ne. wrightj) errorcnt
errorcnt 1 end do
Fortran
SASM
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CS 220 Computer Organization SASM (cont.)

• SASM A simple Abstract Language
• Communication
• Input/Output assume input from keyboard/output
  to screen
• One Input Instruction
• get_ch ch
• Four Output Instructions
• put_ch ch
• put_str stringname
• put_i x
• put_fp fl

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CS 220 Computer Organization SASM (cont.)

• SASM A simple Abstract Language
• Communication (cont.)
• String processing
• strings must be null terminated to use the
  put_str instruction
• Null terminated means to end in a zero
• Example
• errmess db You goofed big time!, 0
• errmess2 db Oh now you did it!, 0ah, 0
  err mess with newline

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CS 220 Computer Organization SASM (cont.)

• Sample Program (Miller, p40)

SASM program to sum the integers 1 through
25 .486 .model flat, stdcall .stack
1000h include sasmacros.inc title sumints
program .data n dd ? number of integers to
sum sum dd ? running sum of integers i dd ? the
integers, loop induction variable str1 db The
sum of the first , 0 str2 db integers is ,
0 newline db 0ah . . .
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CS 220 Computer Organization SASM (cont.)

• Sample Program (Miller, p40)

n dd 25 number of integers to sum sum dd 0 runni
ng sum of integers i dd 1 the integers, loop
induction variable str1 db The sum of the first
, 0 str2 db integers is , 0 newline db 0ah .c
ode for_loop compare i, n loop until i exceeds
n bgz done_for iadd sum, i iadd i,
1 br for_loop done_for put_str str1 print out
results put_i n put_str str2 put_i sum put_ch
newline done end
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CS 220 Computer Organization SASM (cont.)

• In class lab
• Get into groups of 2 to 3. Rearrange the desks
  as necessary.
• Write a SASM program that prints out a sequence
  of n Xs, where n is a positive integer
  initialized in the declarations section of the
  program. (Miller, p42, Problem 14)
• Write out your solution on a sheet of paper. Use
  the figures and examples from the book to help.
  Appendix B, p315 of your text lists the SASM
  instructions.
• Turn in whatever you have completed at the end of
  class. Make sure to sign the names of your
  group.
• These will be graded on your attempt at finding a
  solution, not on the correctness of the solution.

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CS 220 Computer Organization SASM (cont.)

• In class lab
• Prof. Wrights solution to Problem 14,
  chapter 3
• .486
• .model flat, stdcall
• .stack 1000h
• include sasmacros.inc
• title line_of_Xs program
• .data
• n dd 15
• i dd 0
• .code
• main compare i, n
• bez did_it for i 0, n-1
• put_ch X