5.3 Instruction Types
Most computer instructions operate on data; however, there are some that do not. Computer manufacturers regularly group instructions into the following categories:
Data movement instructions are the most frequently used instructions. Data is moved from memory into registers, from registers to registers, and from registers to memory, and many machines provide different instructions depending on the source and destination. For example, there may be a MOVER instruction that always requires two register operands, whereas a MOVE instruction allows one register and one memory operand. Some architectures, such as RISC, limit the instructions that can move data to and from memory in an attempt to speed up execution. Many machines have variations of load, store, and move instructions to handle data of different sizes. For example, there may be a LOADB instruction for dealing with bytes and a LOADW instruction for handling words.
Arithmetic operations include those instructions that use integers and floating point numbers. Many instruction sets provide different arithmetic instructions for various data sizes. As with the data movement instructions, there are sometimes different instructions for providing various combinations of register and memory accesses in different addressing modes.
Boolean logic instructions perform Boolean operations, much in the same way that arithmetic operations work. There are typically instructions for performing AND, NOT, and often OR and XOR operations.
Bit manipulation instructions are used for setting and resetting individual bits (or sometimes groups of bits) within a given data word. These include both arithmetic and logical shift instructions and rotate instructions, both to the left and to the right. Logical shift instructions simply shift bits to either the left or the right by a specified amount, shifting in zeros from the opposite end. Arithmetic shift instructions, commonly used to multiply or divide by 2, do not shift the leftmost bit, because this represents the sign of the number. On a right arithmetic shift, the sign bit is replicated into the bit position to its right. On a left arithmetic shift, values are shifted left, zeros are shifted in, but the sign bit is never moved. Rotate instructions are simply shift instructions that shift in the bits that are shifted out. For example, on a rotate left 1 bit, the leftmost bit is shifted out and rotated around to become the rightmost bit.
I/O instructions vary greatly from architecture to architecture. The basic schemes for handling I/O are programmed I/O, interrupt-driven I/O, and DMA devices. These are covered in more detail in Chapter 7.
Control instructions include branches, skips, and procedure calls. Branching can be unconditional or conditional. Skip instructions are basically branch instructions with implied addresses. Because no operand is required, skip instructions often use bits of the address field to specify different situations (recall the Skipcond instruction used by MARIE). Procedure calls are special branch instructions that automatically save the return address. Different machines use different methods to save this address. Some store the address at a specific location in memory, others store it in a register, while still others push the return address on a stack. We have already seen that stacks can be used for other purposes.
Special purpose instructions include those used for string processing, high-level language support, protection, flag control, and cache management. Most architectures provide instructions for string processing, including string manipulation and searching.
