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instructions are a single word (32 bits) in size. In R/M, the source and destination are the same register, designated Rds1. The R+M includes many more formats; the variety is shown in Figure 1.18. In Figures 1.17 and 1.18, the address represents space for base and index register indicators as well as a memory address or offset value. |
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Even when the format is known, other details, such as the operand addressing mode, may in actual situations determine an instruction's size. For example, |
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OP R1, [R2] |
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may in some R/M or R+M machines be representable in a 16-bit RR format while in other machines be (implicitly) representable as |
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OP R1, 0[R2] |
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that is, in a 32-bit R/M format. We usually assume that such non-direct register mode requires the "RM" format. Thus, in an R/M machine, |
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OP R1, [R2]
OP R1, #valve
LDA R1, disp[Rbase, Rindex] |
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all require the 32-bit RM format. The LDA instruction simply loads the indicated address into R1, where disp indicates an address displacement, Rbase a base register, and Rindex an index register. |
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1.6 Addressing and Memory |
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Addressing in modern machines consists of several levels of address mappings [69, 70, 161]. Each level (the user program, the operating system, or the hardware manager) provides special services for the user. |
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Generally, we can describe three levels of addressing (Figure 1.19): |
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1. The process or user program level. At this level, the primary concern is with the efficient representation of user statements. |
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2. The system level. The primary concern is with multiple processes sharing a fixed address space. Issues include relocation and protection. |
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3. The memory level. This is the set (or sets) of physical locations used to interpret level 1 and 2 addresses. The primary concern is with access time. A memory manager is responsible for the prediction of localities (or working sets) that are about to be used so that they can be readied (accessed) by the processor. |
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The designer must provide the user with facilities for efficiently addressing a large number of objects. In almost all modern processors, the basic |
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