Several times, in earlier chapters, I stated that the ARM architecture was designed with the embedded world in mind. Although the cost of the memory is everyday lower, it still may account as an important part of the budget of an embedded system. The ARM instruction set has several features meant to reduce the impact of code size. One of the features which helps in such approach is predication.

In chapter 9 we were introduced to functions and we saw that they have to follow a number of conventions in order to play nice with other functions. We also briefly mentioned the stack, as an area of memory owned solely by the function. In this chapter we will go in depth with the stack and why it is important for functions.

In previous chapters we learnt the foundations of ARM assembler: registers, some arithmetic operations, loads and stores and branches. Now it is time to put everything together and add another level of abstraction to our assembler skills: functions.

In the previous chapter we saw that the second operand of most arithmetic instructions can use a shift operator which allows us to shift and rotate bits. In this chapter we will continue learning the available indexing modes of ARM instructions. This time we will focus on load and store instructions.

ARM architecture has been for long targeted at embedded systems. Embedded systems usually end being used in massively manufactured products (dishwashers, mobile phones, TV sets, etc). In this context margins are very tight so a designer will always try to spare as much components as possible (a cent saved in hundreds of thousands or even millions of appliances may pay off). One relatively expensive component is memory although every day memory is less and less expensive. Anyway, in constrained memory environments being able to save memory is good and ARM instruction set was designed with this goal in mind. It will take us several chapters to learn all of these techniques, today we will start with one feature usually named shifted operand.