Branch and Subroutine

Summary

This chapter focuses on the program control instructions, which are used to alter the flow of the program by taking a branch or calling a subroutine. It explains branch instruction and conditional branch. The chapter also focuses on indirect branch and its uses. It discusses nested subroutines and an issue that arises when a called subroutine modifies the registers used by the calling subroutine. Programs are usually written in a modular structure, using subroutines as building blocks. A subroutine is a sequence of instructions written to achieve a specific functionality. Rather than repeating the same sequences of instructions multiple times a program, a subroutine allows for a more efficient approach: define once, call whenever needed. A subroutine is organized as a sequence of instructions. When a subroutine calls another subroutine before completing its own execution, then the two subroutines are nested. Indirect branches can be used to implement selection logic and long jumps.