Decision-making in Assembly Language

Abstract

In the previous chapter (section 3.1) reference was made to a new register: P, the status register. Figure 3.1 shows it in relation to the rest of the architecture that we have met so far. Each of the eight bits of this register has a separate function: each operates as a flag, indicating that certain conditions have or have not occurred. If the bit is 1, the flag is said to be set, if the bit is 0, the flag is said to be clear (or sometimes reset). We have already considered two flags: C, the carry flag, and V, the overflow flag. Figure 4.1 shows all the flags in the P register. There are five new ones, N, B, D, I, Z. Of these, we will defer discussion of B, D and I until later chapters.