CSC 314 Notes 
Friday Nov 21, 2003

Topics: 

• Exam #2 Monday Nov 24
• Finish from last time:
• More details about Sun assembly
  • The registers
  • Instructions
  • RISC design
• Embedding Assembly in a C Program

Embedding ASM in C/C++

• writing code is assembly language is hard
  • details, lots of instructions to accomplish small tasks
  • that is why high-level languages were developed
• different rules for different languages (C, C++, ...)
• naming conventions
  • HLL goes through compiler to generated code
  • renaming takes place, must know naming conventions to match things up
• memory model
  • segments, how to refer to memory locations
• calling conventions
  • how to send arguments to functions
  • by value, by reference
  • how things are restored after procedure
  • responsibilities of caller and called
• external identifiers
  • indicate information so linker and put the connections together

Inline Assembly Code

• assembly code inserted directly into HLL programs
• simplicity
  • no need to worry about the issues above
• code optimization
  • attack a computational expensive portion of code (loops)
• lack of portability
  • ASM is platform dependent
  • different languages on different machines
• __asm directive in Visual C++

  • __asm statement

  • __asm {
           stmts
          }

• use instructions and registers
  • but don't assume values in registers
• make use of external (from the asm-block) symbol names
• some limitations
  • mostly regarding assembler directives
  • defining data
• decent examples in the text

Linking to C++ Programs

• write external procedures in assembly that can be called from C and C++ programs
• one module contains the assembly language code that contains the external procedure
• one module contains the C/C++ code that starts and ends the program
• must understand some details about how C++ is translated or implemented
• arguments
  • passed right to left from argument list
  •  calling program must clean up stack upon return
• external names
  • appends _ to beginning of each external identifier

Generated and Optimized Code

• hand written code has the potential to be the most efficient
• naively generated code can be quite inefficient
  • useless operations, repetitive work
• optimizing compiler
  • does good job
  • takes more time

Example on the Sun250 using gcc -- presented in class

Very simple, small example - but it works

#include <stdio.h>

int main()
{
 int x;

 x = 5;


 asm("ld [%fp-12], %g1");
 asm("add %g1, %g1, %g1");
 asm("add %g1, %g1, %g1");
 asm("st %g1, [%fp-12]");


 printf("X is %d\n", x);

 return 0;
}