The asm construct

1
The asm construct

• An introduction to the GNU C/C compilers
  obscure syntax for doing inline assembly language

2
The asm construct

• When using C/C for systems programs, we
  sometimes need to employ processor-specific
  instructions (e.g., to access CPU registers or
  the current stack area)
• Because our high-level languages strive for
  portability across different hardware
  platforms, these languages dont provide direct
  access to CPU registers or stack

3
gcc/g extensions

• The GNU compilers support an extension to the
  language which allows us to insert assembler code
  into our instruction-stream
• Operands in registers or global variables can
  directly appear in assembly language, like this
  (as can immediate operands)
• int count 4 // global variable
• asm( movl count , eax )
• asm( imull 5, eax, ecx )

4
Local variables

• Variables defined as local to a function are more
  awkward to reference by name with the asm
  construct, because they reside on the stack and
  require the generation of offsets from the ebp
  register-contents
• A special syntax is available for handling such
  situations in a manner that gcc/g can decipher

5
Template

• The general construct-format is as follows
• asm( instruction-template
• output-operand
• input-operand
• clobber-list )

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Example from switcher.cpp

• void upon_signal( int signum )
• unsigned long tos
• asm(" movl ebp, 0 " "m" (tos) )
• for (int i 0 i lt 22 i)
• printf( "tosd08X \n", i, tosi )

7
Example from pgfaults.c

• void load_IDTR( void img )
• asm( lidt 0 m (img) )
• Heres how we used this function
• unsigned short newidtr 3
• load_IDTR( newidtr )

8
How to see your results

• You can ask the gcc compiler to stop after
  translating your C/C source-file into x86
  assembly language
• gcc S myprog.cpp
• Then you can view the output myprog.s by using
  the cat command (or an editor)
• cat myprog.s more