CPU Registers
CPU registers are small, high-speed storage locations within the CPU that hold data temporarily during processing. They are used to store instructions, memory addresses, and data for arithmetic operations. Registers are essential for the efficient execution of instructions and play a crucial role in the overall performance of the CPU.
6 Key excerpts on "CPU Registers"
eBook - ePubElectronics Explained
The New Systems Approach to Learning Electronics
- Louis E. Frenzel(Author)
- 2010(Publication Date)
- Newnes(Publisher)
...Outputs go to relays, motors, lights, and liquid crystal displays (LCDs). OPERATIONAL DETAILS Registers and the ALU The main circuit element in a CPU is the register. Usually the data to be processed by the CPU is taken from memory and stored temporarily in a register. Like a memory location, a register usually has a fixed length such as 8, 16, or 32 bits. But unlike a memory location, registers are often used to manipulate data as well as store it. That is, the register can alter or process the data in some way. Examples of how a register can process data are shown in Figure 6.5. A binary word in a register can be shifted one or more bit positions to the right or left, or the register may be connected as an up/down counter so that it can be incremented (add one to the content) or decremented (subtract one from the content). A register can also be reset or cleared, thereby erasing any data in it and leaving the content zero. Data transfers and manipulations performed on a register are initiated by individual computer instructions. For example, one instruction may cause the register to be loaded from a memory location. Another instruction may cause the word in the register to be transferred to a memory location. All data transfers are parallel, meaning that all bits are moved simultaneously from the source to the destination. The main working register in most digital computers is called the accumulator. It may also be called the A register, W register, or something else. FIGURE 6.5 How data is manipulated in a register. Examples of shifting data, incrementing and decrementing a register, and clearing (resetting) a register to 0. The accumulator can hold one word whose bit length is equal to that of a memory location. An 8-bit microprocessor has an 8-bit accumulator. The accumulator is part of the ALU. The accumulator can be loaded from memory or the accumulator content can be stored in any memory location...
- Ed Lipiansky(Author)
- 2012(Publication Date)
- Wiley-IEEE Press(Publisher)
...The CPU Registers are part the programmer’s model of the CPU. However, there will invariably be other registers, mechanisms and devices that are totally transparent (or not visible) to the programmer. So before looking at the big view of the data path we will study bits and pieces of the fundamental elements that constitute such data path architecture. 10.3.1 The Registers Our CPU needs registers. Registers are used to hold memory addresses, memory data read or memory data to be written. Registers also hold the operand of the ADD and AND instructions. Registers are typically built with D type flip-flops; however they are not just a free running group of flip-flops. Why not? If the registers were free running they would get loaded on every single active edge of the clock. To achieve a selective load of a register we place a 1-of-2 Mux as shown in Figure 10.1. When the CPU control circuit drives a zero onto the mux select line, the register holds it data indefinitely because it keeps reloading itself with its own outputs for as long as clocks keep coming into the register. When a control circuit drives a one onto the mux select line, the external data placed on channel 1 of the mux gets loaded upon the active edge of the clock clocking the register. Figure 10.1 Implementation of a register with synchronous load control. For example if we have a data path like the one shown in Figure 10.2, which has four 8-bit registers A, B, C, and D. Assume that somehow all four registers have their own initial values. Let us assume that on the next clock edge we would like the contents of register A to get transferred to register B, overwriting the current value of B and preserving the current values of A, C, and D. The control required to do that has to assert the load input for register B and keep the load inputs of registers A, C, and D negated. The mux select lines have to select input channel 0, which feeds the contents of register A onto the inputs of all four registers...
eBook - ePub- J S Anderson(Author)
- 2012(Publication Date)
- Routledge(Publisher)
...A CMOS device running from a single 5V supply; pin compatible with 64K EPROMS, i.e. 64K bits. We have described memory chips (RAM and ROM, etc.) as being composed of basic bistable units, generally arranged in groups of eight each capable of holding a single byte of data. A similar arrangement is used in the composition of a register. This can be regarded as essentially the same thing. The CPU has several of these registers within its structure which enable it to hold numbers (as a binary sequence) for processing. Figure 3.2 shows a simple representation of a Z80 CPU, the most important parts of which are the instruction register, flag register, the other ‘general purpose’ registers, the ALU and the program counter. In Figure 3.3 we show a simple representation of the 6502 CPU (as used in the PET, APPLE, ‘EMMA’ and BBC computers). The essential features are the same; there are fewer registers (only two apart from the accumulator) and, although it does exactly the same thing, the flag register is called the status register. Other 8-bit CPUs include the Intel 8080 and 8085, and the Motorola 6800. Generally, a microprocessor contains: 1 an instruction register (IR); 2 an arithmetic and logic unit (ALU); 3 a set of ‘general purpose’ registers; 4 a set of ‘special purpose’ registers; 5 control and timing devices. Control, address and data connections, internally and externally, are made via a system of wires called buses’. The data and control buses in 6502 and Z80 systems have 8 lines, whilst the address bus has 16 lines. The 6502 CPU has three general purpose registers: an accumulator and two index registers, X and Y. Figure 3.2 The Z80 central processing unit Figure 3.3 The 6502 central processing unit The flag register is called the processor status register and the stack register is 8 bits wide. Otherwise, as far as registers are concerned, the 6502 is very similar to the Z80...
- Mohamed Rafiquzzaman(Author)
- 2021(Publication Date)
- CRC Press(Publisher)
...The 32-bit microprocessors include special on-chip combinational network called the Barrel Shifter. The PC normally contains the address of the next instruction to be executed. Upon activating the microprocessor chip’s RESET input, the PC is normally initialized with the address of the first instruction. For example, the 80486, upon hardware reset, reads the first instruction from the 32-bit address FFFFFFF0 16. In order to execute the instruction, the microprocessor normally places the PC contents on the address bus and reads (fetches) the first instruction from external memory. The program counter contents are then automatically incremented by the ALU. The microcomputer thus executes a program sequentially unless it encounters a jump or branch instruction. The size of the PC varies from one microprocessor to another depending on the address size. For example, the 68000 has a 24-bit PC, while the 68040 contains a 32-bit PC. The instruction register (IR) contains the instruction to be executed. After fetching an instruction from memory, the microprocessor places it in the IR for translation. The status register contains individual bits with each bit having a special meaning. The bits in the status register are called flags. Each flag is usually set or reset by an ALU operation. The flags are used by the Conditional Branch instructions. Typical flags include carry, sign, zero, and overflow. The carry (C) flag is used to reflect whether or not an arithmetic operation such as ADD generates a carry. The carry is generated out of the 8th bit (bit 7) for byte operations, 16th bit (bit 15) for 16-bit, or 32nd bit (bit 31) for 32-bit operations. The carry is used as the borrow flag for subtraction. In multiple word arithmetic operations, any carry from a low-order word must be reflected in the high-order word for correct results. The zero (Z) flag is used to indicate whether the result of an arithmetic or logic operation is zero...
eBook - ePub- K.L.S. Sharma(Author)
- 2016(Publication Date)
- Elsevier(Publisher)
...Upon completing executing every instruction, the program counter increments by either 1 or 3 automatically (see Section C.2.4) to indicate the address of the memory location of the next instruction in the program. Figure C.1 Address distribution. • Accumulator: A register to perform the following functions: • Hold the data read from memory/registers temporarily during its manipulation • Hold the result before transferring it to memory/registers • Act as one of the two operands in arithmetic/logical operations C.2.3. Data Range in Memory/Registers As illustrated in Fig. C.2, the processor, an 8-bit machine, can accommodate a maximum of 8 bits (1 byte) of information content or supporting data that range as follows: Figure C.2 Data range in memory/registers. • Binary, 00 000 000 to 11 111 111 • Octal, 000 to 377 • Decimal, 0 to 255 The content of a byte in the memory can be an instruction, an address, or data (arithmetic or logical) whereas the content in a register is always the data. C.2.4. Instruction Formats and Sets Here basic instruction examples have been selected to address the concepts in the working of processor for automation strategy programming. In practice, the processors have an extensive repertoire of instructions. For the program (instructions stored in the memory) to operate on the data in the memory/registers of the functional modules, we need to define the instructions for various operations for data movement, logical, arithmetic, and control operations. The following sections illustrate the various instruction formats. The general format of the instruction has two fields, operation code and address. Furthermore, some instructions do not need the address field whereas others need one (2 bytes), as illustrated in Fig. C.3. Figure C.3 Instruction formats. Instructions without address fields occupy one location in the program memory and make the program counter increment by 1; upon completion they go to the next instruction in the program...
eBook - ePub- David Johnson(Author)
- 2020(Publication Date)
- CRC Press(Publisher)
...4 The Central Processing Unit Although referred to as the brain of the system, the Central Processing Unit in a normal installation is the unsung hero, buried in a control cabinet, all but forgotten. 4.1 BASIC FUNCTIONALITY In a programmable controller system, the central processing unit (CPU) provides both the heart and the brain required for successful and timely control execution. It rapidly and efficiently scans all of the system inputs, examines and solves the application logic, and updates all of the system outputs. In addition, it also gives itself a checkup each scan to ensure that its structure is still intact. In this chapter we will examine the central processing unit as it relates to the entire system. Included will be the various functional blocks in the CPU, typical scan techniques, I/O interface and memory uses, power supplies, and system diagnostics. 4.2 TYPICAL FUNCTION BLOCK INTERACTIONS In practice, the central processing unit can vary in its architecture, but consists of the basic building block structure illustrated in Figure 4.1. The processor section consists of one or more microprocessors and their associated circuitry. While it is true that some of the older generation programmable controllers were designed without the luxury of using microprocessors, most modern systems use either a single microprocessor such as the 8086 or Z-80, or multiple microprocessors such as the AMD2903, used in a bit slice architecture. This multi-tasking approach is used in the multiple microprocessor system to break the control system tasks into many small components which can be executed in parallel. The result of this approach is to achieve execution speeds that are orders of magnitude faster than their single-tasking counterparts...
