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2. The R/M, or register-memory machines, which include the IBM System/390 and, more loosely, the Intel x86 series of instruction sets. |
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3. The R+M, or register-plus-memory architecture, which includes the DEC VAX and, more loosely, the Motorola 680´0 class of machines. |
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While the partitioning is only approximate, it is important to recognize that fundamental differences in instruction set formats, as represented by these classes, lead to significantly different levels of code density and implementation tradeoffs. |
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In dealing with addressing and memory, three levels of address mapping can be distinguished: |
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1. The processor, or user level, which is the address space visible to the user. |
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2. The system address space, which is the address space visible to the operating system. |
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3. The memory space, which is the address space visible to the hardware. |
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These three levels of mapping involve different tradeoffs and different considerations that must be carefully integrated for an efficient overall mapping mechanism. |
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Given a well-defined instruction set and the basic timing parameters of a processor (internal cycle time, definition of the cycle, and memory timing considerations), it is generally possible to predict instruction timing. There is an expected sequence of actions associated with the interpretation of each instruction. When an implementation executes the expected sequence, we describe that implementation as a well-mapped design. |
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1.10 Historical Development of Computers by Format Class |
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In terms of modern machines, the instruction set that has had the most impact has been IBM's System/360 [40, 41], which dominates the mainframe marketplace even today as IBM's System/390. It is our prototypical R/M machine. System/360 introduced the notion of general-purpose registers and multiple registers to hold fixed-point register values and addresses, coupled with a separate set of multiple registers to hold floating-point values. Other innovations of the day were byte-addressing, use of a program status word, condition codes, and user-system states. While a number of these items can be found in vestigial form in even earlier machines, System/360 with its market dominance organized these concepts and influenced later instruction set designers in significant ways. |
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