(The Morgan Kaufmann Series in Computer Architecture and Design) David A. Patterson, John L. Hennessy - Computer Organization and Design RISC-V Edition_ The Hardware Software Interface-Morgan Kaufmann-102-258-pages-6.pdf

2.6 Logical Operations 95 What RISC-V instruction does this represent? Choose from one of the four options below....

2.6 Logical Operations 95 What RISC-V instruction does this represent? Choose from one of the four options below. Check Yourself funct7 rs2 rs1 funct3 rd opcode 32 9 10 000 11 51 1. sub x9, x10, x11 2. add x11, x9, x10 3. sub x11, x10, x9 4. sub x11, x9, x10 If a person is age 40ten, what is their age in hexidecimal? 2.6 Logical Operations “Contrariwise,” Although the first computers operated on full words, it soon became clear that continued Tweedledee, it was useful to operate on fields of bits within a word or even on individual bits. “if it was so, it might Examining characters within a word, each of which is stored as 8 bits, is one example be; and if it were so, it of such an operation (see Section 2.9). It follows that operations were added to would be; but as programming languages and instruction set architectures to simplify, among other it isn’t, it ain’t. things, the packing and unpacking of bits into words. These instructions are called That’s logic.” logical operations. Figure 2.8 shows logical operations in C, Java, and RISC-V. Lewis Carroll, Alice’s Adventures in Wonderland, 1865 FIGURE 2.8 C and Java logical operators and their corresponding RISC-V instructions. One way to implement NOT is to use XOR with one operand being all ones (FFFF FFFF FFFF FFFFhex). 96 Chapter 2 Instructions: Language of the Computer The first class of such operations is called shifts. They move all the bits in a word to the left or right, filling the emptied bits with 0s. For example, if register x19 contained 00000000 00000000 00000000 00001001two = 9ten and the instruction to shift left by 4 was executed, the new value would be: 00000000 00000000 00000000 10010000two = 144ten The dual of a shift left is a shift right. The actual names of the two RISC-V shift instructions are shift left logical immediate (slli) and shift right logical immediate (srli). The following instruction performs the operation above, if the original value was in register x19 and the result should go in register x11: slli x11, x19, 4 // reg x11 = reg x19