Hardware and Computer Organization- P11:Today, we often take for granted the impressive array of computing machinery that surrounds us and helps us manage our daily lives. Because you are studying computer architecture and digital hardware, you no doubt have a good understanding of these machines, and you’ve probably written countless programs on your PCs and workstations. | Chapter 10 We see this in the next two bytes of the instruction AA 00. The r m field value 001 then gives us the final form of the address calculation BX DI DISP . The final byte of the instruction 55h is the 8-bit immediate value imm8 as shown in the format of the opcode. If we modify the instruction as follows CS MOV BX DI 0AAh 5555h The instruction code becomes 2E C7 81 AA 00 55 55. The opcode changes from C6 to C7 indicating a word operation and the immediate field is now two bytes long. The entire instruction is now 7 bytes long so we can see why the 8086 architecture must allow for nonaligned accesses from memory since the next instruction would be starting on an odd boundary if this instruction began on a word boundary. 8086 Instruction Set Summary While it is beyond the scope of this text to cover the 8086 family instructions in detail we can look at representative instructions in each class and use these instructions as to gain an understanding of all of the instructions. Also some of the instructions will also extend our insight into the overall x86 family architecture. We can classify the original x86 instruction set into the following instruction families Data transfer instructions Arithmetic instructions Logic instructions String manipulation instructions Control transfer instructions Most of these instruction classes should look familiar to you. Others are new. The 68K family instruction set did not have dedicated instructions for loop control or string manipulation. Whether or not this represents a shortcoming of the architecture is difficult to assess. Clearly you are able to manipulate strings and form software loops in the 68K architecture as well. Let s now look at some representative instructions in each of these classes. Data Transfer Instructions Just as with the MOVE instruction of the 68K family the MOV instruction is probably the most oft-used instruction in the instruction set. We ve looked at the MOV instruction in some detail in this .
