I have been reverse-engineering the Z processor using images from the Visual team. The image below shows the overall structure of the Z chip and the location of the ALU. The remainder of this article dives into the details of the ALU: its architecture, how it works, and exactly how it is implemented. I've created the following block diagram to give an overview of the structure of the Z's ALU. Unlike Z block diagrams published elsewhere, this block diagram is based on the actual silicon.

At the left of the diagram, the register bus provides the ALU's connection to the register file and the rest of the CPU. The operation of the ALU starts by loading two 8-bit operands from registers into internal latches. The ALU does a computation on the low 4 bits of the operands and stores the result internally in latches.

Next the ALU processes the high 4 bits of the operands. Finally, the ALU writes the 8 bits of result the 4 low bits from the latch, and the 4 high bits just computed back to the registers. Thus, by doing two computation cycles, the ALU is able to process a full 8 bits of data. As the block diagram shows, the ALU has two internal 4-bit buses connected to the 8-bit register bus: the low bus provides access to bits 0, 1, 2, and 3 of registers, while the high bus provides access to bits 4, 5, 6, and 7.

The ALU uses latches to store the operands until it can use them.

### 32-bit computing

The op1 latches hold the first operand, and the op2 latches hold the second operand. Each operand has 4 bits of low latch and 4 bits of high latch, to store 8 bits. Multiplexers select which data is used for the computation. The op1 latches are connected to a multiplexer that selects either the low or high four bits. The op2 latches are connected to a multiplexer that selects either the low or high four bits, as well as selecting either the value or the inverted value. The inverted value is used for subtraction, negation, and comparison.

The ALU first performs one computation on the low bits, storing the 4-bit result into the result low latch. The ALU then performs a second computation on the high bits, writing the latched low result and the freshly-computed high bits back to the bus.

The carry from the first computation is used in the second computation if needed. The Z provides extensive bit-addressed operations, allowing a single bit in a byte to be set, reset, or tested. In a bit-addressed operation, bits 5, 4, and 3 of the instruction select which of the 8 bits to use.

On the far right of the ALU block diagram is the bit select circuit that support these operations. In this circuit, simple logic gates select one of eight bits based on the instruction. The 8-bit result is written to the ALU bus, where it is used for the bit-addressed operation. Thus, decoding this part of an instruction happens right at the ALU, rather than in the regular instruction decode logic.

The Z's shift circuitry is interesting.An arithmetic logic unit ALU is a combinational digital electronic circuit that performs arithmetic and bitwise operations on integer binary numbers. This is in contrast to a floating-point unit FPUwhich operates on floating point numbers. The inputs to an ALU are the data to be operated on, called operandsand a code indicating the operation to be performed; the ALU's output is the result of the performed operation.

In many designs, the ALU also has status inputs or outputs, or both, which convey information about a previous operation or the current operation, respectively, between the ALU and external status registers. An ALU has a variety of input and output netswhich are the electrical conductors used to convey digital signals between the ALU and external circuitry. When an ALU is operating, external circuits apply signals to the ALU inputs and, in response, the ALU produces and conveys signals to external circuitry via its outputs.

Each data bus is a group of signals that conveys one binary integer number. Typically, the A, B and Y bus widths the number of signals comprising each bus are identical and match the native word size of the external circuitry e. The opcode input is a parallel bus that conveys to the ALU an operation selection code, which is an enumerated value that specifies the desired arithmetic or logic operation to be performed by the ALU.

The opcode size its bus width determines the maximum number of different operations the ALU can perform; for example, a four-bit opcode can specify up to sixteen different ALU operations. Generally, an ALU opcode is not the same as a machine language opcodethough in some cases it may be directly encoded as a bit field within a machine language opcode. The status outputs are various individual signals that convey supplemental information about the result of the current ALU operation.

General-purpose ALUs commonly have status signals such as:. At the end of each ALU operation, the status output signals are usually stored in external registers to make them available for future ALU operations e.

The collection of bit registers that store the status outputs are often treated as a single, multi-bit register, which is referred to as the "status register" or "condition code register". The status inputs allow additional information to be made available to the ALU when performing an operation. Typically, this is a single "carry-in" bit that is the stored carry-out from a previous ALU operation. An ALU is a combinational logic circuit, meaning that its outputs will change asynchronously in response to input changes.

In normal operation, stable signals are applied to all of the ALU inputs and, when enough time known as the " propagation delay " has passed for the signals to propagate through the ALU circuitry, the result of the ALU operation appears at the ALU outputs. The external circuitry connected to the ALU is responsible for ensuring the stability of ALU input signals throughout the operation, and for allowing sufficient time for the signals to propagate through the ALU before sampling the ALU result.Also, bit CPU and ALU architectures are those that are based on registersaddress busesor data buses of that size.

A bit register can store 2 32 different values. The range of integer values that can be stored in 32 bits depends on the integer representation used. The world's first stored program electronic computer, the Manchester Babyused a bit architecture inalthough it was only a proof of concept and had little practical capacity.

It held only 32 bit words of RAM on a Williams tubeand had no addition operation, only subtraction. The bit processor was the primary processor used in all computers until the early s. Memory, as well as other digital circuits and wiring, was expensive during the first decades of bit architectures the s to the s.

This could be a bit ALUfor instance, or external or internal buses narrower than 32 bits, limiting memory size or demanding more cycles for instruction fetch, execution or write back. Despite this, such processors could be labeled "bit," since they still had bit registers and instructions able to manipulate bit quantities.

However, the opposite is often true for newer bit designs. On the x86 architecturea bit application normally means software that typically not necessarily uses the bit linear address space or flat memory model possible with the and later chips.

As this is quite time-consuming in comparison to other machine operations, the performance may suffer. Furthermore, programming with segments tend to become complicated; special far and near keywords or memory models had to be used with carenot only in assembly language but also in high level languages such as Pascalcompiled BASICFortranCetc.

The and its successors fully support the bit segments of the but also segments for bit address offsets using the new bit width of the main registers. If the base address of all bit segments is set to 0, and segment registers are not used explicitly, the segmentation can be forgotten and the processor appears as having a simple linear bit address space.

The former possibility exists for backward compatibility and the latter is usually meant to be used for new software development. Other image formats also specify 32 bits per pixel, such as RGBE. In digital images, bit sometimes refers to high-dynamic-range imaging HDR formats that use 32 bits per channel, a total of 96 bits per pixel. For example, a reflection in an oil slick is only a fraction of that seen in a mirror surface.

HDR imagery allows for the reflection of highlights that can still be seen as bright white areas, instead of dull grey shapes. A bit file format is a binary file format for which each elementary information is defined on 32 bits or 4 bytes.

## How to Build Your Own Discrete 4-Bit ALU

An example of such a format is the Enhanced Metafile Format. From Wikipedia, the free encyclopedia. Computer architecture. This article needs additional citations for verification.Skip to Main Content. A not-for-profit organization, IEEE is the world's largest technical professional organization dedicated to advancing technology for the benefit of humanity. Use of this web site signifies your agreement to the terms and conditions.

**Four bit Arithmetic Logic Unit Design Isis Simulation**

Personal Sign In. For IEEE to continue sending you helpful information on our products and services, please consent to our updated Privacy Policy. Email Address. Sign In. Access provided by: anon Sign Out. It processes bit-sliced bit data that are divided into eight slices of 4 bits. The bit-slice approach simplifies the circuit structure and reduces the hardware cost.

The ALU uses synchronous concurrent-flow clocking and consists of eight pipeline stages. It was implemented using the 1. It consists of Josephson junctions with an area of 3. It achieved the target frequency of 50 GHz and a latency of ps for a bit operation, at the designed DC bias voltage of 2.

Furthermore, it achieved a throughput of 6. The proposed ALU can be used for any 4n-bit processing. Article : Date of Publication: 09 December DOI: Need Help?An Arithmetic Logic Unit ALU is a combinational circuit that performs logical and arithmetic operations on a pair of n-bit operands.

The inputs a and b are signed, two's complement numbers when they are presented to the input of the ALU. The operations performed by an ALU are controlled by a set of operation-select inputs. In this lab, you will design a 4-bit ALU with 2 operation-select inputs, aluop[]. Logical operations take place on the bits that comprise a value known as bitwise operationswhile arithmetic operations treat inputs and outputs as two's complement integers. If an addition results in overflow, enable the Overflow output.

The different operations will be selected by a 2-bit control signal called " aluop " according to the following Table. In a bottom-up design flow, the entire design is compiled in separate projects and locked down once the designer has achieved timing closure on the blocks. The lower-level partitions are then imported into the top-level project for final integration. Figure 1 shows the diagram of the basic idea for a 1-bit ALU: produce the different results in parallel, then select one according to the opcode with a multiplexer.

Each of the OP boxes computes 1-bit for an operation based on the requirement. The multiplexer selects the output of the appropriate 1-bit operation. Its control input aluop is derived from operation and function bits of a machine instruction.

When two bits a and b are added, a sum and a carry are generated. A combinational circuit that adds two bits only is called a "half adder". However, after adding the least significant bits bit 0 of the two numbers, the carrying value from the adder needs to be added into the next bit of numbers. On the other hand, addition of three bits two bits of the two numbers and a previous carry, which may be 0 or 1 is required for all the subsequent bits.

A combinational circuit that adds three bits, generating a sum and a carry which may be 0 or 1is called a "full adder. Figure 2 shows the block diagram of a full adder. The full adder adds three bits aband cin and generates a sum bit and a carry bit s and cout. The 1-bit full adder only does single-digit addition. Multiple copies can be used to make adders for any size of binary numbers.

By default, the carry-in to the lowest bit adder LSb, bit 0 is 0. Carry-out of one digit's adder becomes the carry-in to the next highest digit's adder. The carry-out of the most significant bit adder is the carry-out of the entire operation. The 4-bit adder is shown in the following figure. Figure 3 : A 4-bit Binary Adder. We already design full-adder in the circuit, we can use full adder to implement binary subtraction simply by using two's complement on the subtrahend b inputs.Arithmetic and Logic Unit in Detail.

In this tutorial we will look at what an ALU really is? We will discuss a 4 bit ALU; this would limit many possibilities We would assume that associated registers and instruction set are also 4 bit. Lets start with a simple half adder.

Half adder adds two single binary digits A and B. It has two outputs, sum S and carry C. The carry signal represents an overflow into the next digit of a multi-digit addition.

Figures below illustrate a simple half adder constructed from logic gates. Full Adder is an extension of half adder to include the Cin input as well. The truth table can be implemented to form the logic diagram as shown below. The multiplexer selects only one operation at a time. The operation selected depends on the selection lines of the multiplexer as shown in the truth table.

Thus a single building block can be constructed and used recursively. The inputs A and B are four bits and the output is 4 bit as well. Figure below illustrates it:. There are a few important takeaways here:. Please enable JavaScript to view the comments powered by Disqus. Log in.

ALU in Detail. These selection lines combined with the input arguments and desired functions a Instruction Set can be formed. These Instructions can used to create meaningful programs. Since these are required to be easily available they can be stored on ROM unit. The input arguments A and B are often stored in Internal Registers. These along with other special purpose register form the registers of the microcontroller.I guess you did not "Zoom Fit" to see the whole simulation waveform. Simulation waveform for the ALU:.

What is an FPGA? Verilog code for FIFO memory 3. Verilog code for bit single-cycle MIPS processor 4. Verilog code for basic logic components in digital circuits 6.

Verilog code for bit Unsigned Divider 7. Verilog code for Fixed-Point Matrix Multiplication 8. Verilog code for Carry-Look-Ahead Multiplier Verilog code for a Microcontroller Verilog code for 4x4 Multiplier Verilog code for Car Parking System Verilog code for Traffic Light Controller Verilog code for comparator design Verilog code for D Flip Flop Verilog code for Full Adder Verilog code for counter with testbench Verilog code for button debouncing on FPGA Newer Post Older Post Home.

Subscribe to: Post Comments Atom. Today, f Verilog code for counter with testbench. In this project, Verilog code for counters with testbench will be presented including up counter, down counter, up-down counter, and r Verilog code for bit single cycle MIPS processor. Verilog code for D Flip Flop. D Flip-Flop is a fundamental component in digital logic circuits. Verilog code for D Flip Flop is presented in this project.

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