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sity of SRAM as compared to DRAM. An SRAM-based main memory requires more devices, more
circuit board area, and more connecting wiresall requirements that add cost and reduce the reliability of a system. Some supercomputers have been built with main memor
tion. Spatial locality says that, if a memory location is accessed, it is likely that nearby locations will
be accessed in the near future. When a microprocessor fetches an instruction, there is a high probability that it will soon fetch the instructions
cute simple instructions at a high rateperhaps one instruction per cycle. Others believe that a microprocessor should execute more complex instructions at a lower rate.
Operand types add complexity to an instruction set when a single general operation suc
sor, it must be able to rapidly fetch instructions, because several RISC instructions are necessary to
match the capabilities of certain CISC instructions. An older computer architecture with an asynchronous memory interface may not be able to provide suf
In contrast to the 68000s CISC architecture, the MIPS family of microprocessors is one of the
commercial pioneers of RISC. MIPS began as a 32-bit architecture with 32-bit instruction words and
32 general-purpose registers. In the 1990s the architecture wa
Computer architecture is central to the design of digital systems, because most digital systems are, at
their core, computers surrounded by varying mixes of interfaces to the outside world. It is difcult to
8-bit read (even byte, A0=0)
8-bit write (odd byte, A0=1)
FIGURE 6.12 68000 asynchronous bus timing.
Little-Endian vs. Big-Endian
memory as you would read and interpret it. T
FIGURE 6.13 68000 synchronous bus timing.
roughly the same time as the data strobes. Therefore, the bus interface logic must make its decisio
a powerful instruction that requires signicant decode logic behind it. Additionally, when opmode
indicates an ADD or ADDX instruction, the two mode values that normally indicate simple register
references now map to one of two special ADDX operations.
line is clean or dirty. When the line is eventually ushed, dirty lines must be written back to main
memory in their entirety. Clean lines can be ushed without further action. While a write-back cache
cannot absolutely eliminate the longer write latency of
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Scientists say that the majori
In 1970s, computer has been involved in the field of art and design and digital technology has a
quickly evolution. Then digital media has a rapid development based on the development of
technology. In 1990s, digital technology was more mature and compute
dled on a process basis rather than an application basis, because it is possible for an application to
consist of multiple semi-independent processes. The high-order address bits referenced by each instruction form the virtual page number (VPN). The PID a
often operate on larger sets of data or that must run many applications simultaneously may merit
larger caches to offset less optimal locality properties. Computers meant to function as computation
engines and network le servers can include several megaby
(L1) cache. The L1 cache is fairly small, anywhere from 2 kB to 64 kB, with its benet being speed.
Because it is small and close to the microprocessor, it can be made to run as fast as the microprocessor can fetch instructions and data. Instruction and da
registers are unlike main memory, because memory just holds data and cannot modify it or take actions based on it.
Whereas caching an I/O region can cause system disruption, caching certain legitimate main memory regions can cause performance degradation
slightly more complex for a four-way set associative cache that would require two status bits per
CACHES IN PRACTICE
Basic cache structures can be applied and augmented in different ways to improve their efcacy. One
common manner in which
ing logic, and each match signal must be logically combined in a single location to generate a nal
hit/miss status ag.
A direct mapped cache, shown in Fig. 7.3, breaks the address bus into three sections; the lower
bits retain their index function within
FIGURE 7.4 128-kB two-way set associative cache.
cache has expanded to
MOTOROLA 68000 16/32-BIT MICROPROCESSOR FAMILY
Motorola followed its 6800 family by leaping directly to a hybrid 16/32-bit microprocessor architecture. Introduced in 1979, the 68000 is a 16-bit microprocessor, due to its 16-bit ALU, but it contains
Despite its complexity and 16-bit processing capability, the 8086 was originally housed in a 40pin DIPthe same package used for most 8-bit processors of the time. Intel chose to use a multiplexed address/data scheme similar to that used on the 8051 microc
cations in which only microamps of current are consumed. To further reduce cost and complexity,
the microcontrollers contain on-board clock drivers that work with a variety of external frequencyreference components. Quartz crystals are supported, as they
serial peripheral bus
FIGURE 5.18 Generic interchip serial bus topology.
require 16 or more signal pins with a byte-wide parallel interface. Not only is
Instructive Microprocessors and
Microprocessors, the heart of digital computers, have been in a constant state of evolution since Intel
developed the rst general-purpose microprocessors in the early 1970s. Intels four-bit
ist today. The 8080 was housed in a 40-pin DIP, featured a 16-bit address bus and an 8-bit data bus,
and ran at 2 MHz. It also implemented a conventional stack pointer that enabled deep stacks in external memory (Intels earlier microprocessors had interna
implemented in the 6800, as shown in Fig. 6.1: a program counter (PC), stack pointer (SP), index
register (X), two general-purpose accumulators (ACCA and ACCB), and status ags set by the ALU
in the condition code register (CCR). ACCA is the primary accumu