Libraries (cont.)

Two types of libraries: static libraries, and dynamic (aka "shared") libraries

• Static library: becomes part of executable
• Dynamic/shared library: exists as a separate file, is not included in executable

Advantages and disadvantages of using static vs. dynamic libraries

Static library:

• advantage: easier to use
• advantage: always the correct version of the library
• disadvantage: makes the executable larger
• disadvantage: updating static library requires rebuilding executable

Dynamic library:

• advantage: smaller executable
• advantage: updating dynamic lib doesn't require updating the executable (usually the case, unless you update the function signatures in the dynamic library)
• advantage: one dynamic library can be shared by many applications
• disadvantage: can complicate version control

System calls

• Programs have limited rights on our systems
• e.g., our programs do not have rights to perform input and output themselves
• a program can ask the OS to do a job (like input/output) on its behalf.
  • -> Such a concept is called a "system call"

Appendix A in the textbook has a table of syscalls, with the following terminology:

• Service: things that the OS offers (e.g., print_string, read_int)
• System call code: e.g., 1, 2, ..., 17
• Arguments: register and datatype, e.g. "$a0 = integer"
• Result: eg., "integer in $v0"

Example: print "a":

addi $v0, $zero, 11 # 11 is the syscall code for 'print'
addi $a0, $zero, 97 # char code for 'a'
syscall

Note: $v0 contains the kind of service that will be used by the syscall (or rather, the code for that service).

Note: this is exam material.