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Category: Z80 logisim

I have been reverse-engineering the Z processor using images from the Visual team.

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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. 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. The and have an additional ALU operation for shift right, and perform shift left by adding the number to itself.

The Z in comparison performs a shift while loading a value into the ALU.

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While the Z reads a value from the register bus, the shift circuit selects which lines from the register bus to use. The circuit loads the value unchanged, shifted left one bit, or shifted right one bit. Values shifted in to bit 0 and 7 are handled separately, since they depend on the specific instruction.

The block diagram also shows a path from the low bus to the high op2 latch, and from the high bus to the low op1 latch. Not shown in the block diagram are the simple circuits to compute parity, test for zero, and check if a 4-bit value is less than These values are used to set the condition flags. The four horizontal "slices" are visible. The organization of each slice approximately matches the block diagram.

The register bus is visible on the left, running vertically with the shifter inputs sticking out from the ALU like "fingers" to obtain the desired bits.By using our site, you acknowledge that you have read and understand our Cookie PolicyPrivacy Policyand our Terms of Service.

Electrical Engineering Stack Exchange is a question and answer site for electronics and electrical engineering professionals, students, and enthusiasts. It only takes a minute to sign up. I've read Code by Charles Petzold, and have begun reading the Microprocessor Design Wikibook which seems to be incomplete. Chapter 17 in Code details the CPU I want to build, but the circuits lack key components -- clock signals, and instruction decoding.

Some of the clock signals seem to be pretty obvious the PC seems to need a steady clock signal but others like how to latch RAM values I have had to think through and try to get working. I can build a working accumulator it can't be accurately called an ALU, I think, because it lacks the L part that switches between addition and subtraction with a single input, and I understand this is all I need for the arithmetic part -- once I get jump opcodes working, I can implement multiplication and division in code.

The part I'm struggling with is the instruction decoding.

z80 logisim

Through some google searches, I see that each opcode needs to be interpreted as multiple microinstructions, but I'm lost as to how I need this to work. Currently, my instruction decoder is just a combinational analysis circuit with a single binary output for each opcode -- 13 in all.

The way the code works is it has one 8 bit code value I only use the low-end byteand then two separate 8 bit address values that I then combine to be the 16 bit address input to the RAM.

In order to latch the values, I have a separate counter that counts up to 10b and then resets to 00b. It is the clock input for each latch in turn for the three latches, there's a, b, and c. I feel like I'm missing some big information regarding instruction decoding and how I need to do clock signals Here are the LogiSim.

Hate to post a "link only" like answer, but I think you should be made aware of Warren Toomey's work with CPU's in Logisim, since it's probably exactly what you are looking for. Though it's not done in Logisim, it is quite well documented, including pictures, schematic, description.

It is also more than just a CPU in a simulator. It's also has the distinct advantage of actually having been built in hardware. In fact, it's currently up and running and serving web pages! Finally The Elements of Computing Systems and the associated site nand2tetris.

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Much All? YouTube would agree; many people have made projects starting from this one source. I think you're missing a key aspect of how ALUs work. Usually each bit of the accumulator is connected to each of the various function blocks via a demultiplexer. Using a command byte, the function is selected and each bit of the accumulator is connected to the proper input of the function block.

The size of the demultiplexer determines how many functions the ALU can handle. In my very crude example shown below, an ALU with a 4 bit input could reference 16 different functions using the demultiplexer:. Note that in most CPUs, this design is optimized into a mess of gates to reduce the transistor count. More functions can be used by having a larger command register, but this would also require more clock cycles to load the command.

I highly recommend reading the following book if you would like to learn more about digital design: Fundamentals of Logic Design 7th ed. When you are planning your micro-instructions you will need to clearly define in your own mind the "traffic" pattern for your data movement through your various blocks. Performing an ADD will require multiple steps.

z80 logisim

If you have holding registers for your two ALU operands then those will need to be loaded from RAM, or some register or bus. If you have a shared internal bus then you may need to load one operand at a time. Once you know what bytes address, immediate literal, RAM data, pointer need to move where, plan the order of the bytes' movement through the bus es into and out of the various registers.

You may get stuck and need to add an internal holding register because some intermediate value must be preserved until the next micro-instruction step. I think the thing you suspect is missing is the complexity required for the logic to take the instruction or opcode and translate it into multiple states inside your CPU.

It can be complex, but it is possible to create a somewhat simple state machine then expand on it with basic logic concepts. For example let's say you are creating the micro-instruction steps for a MOVE operation.Welcome, Guest. Please login or register. Did you miss your activation email? This topic This board Entire forum Google Bing. Print Search. Author Topic: How do I make a 8 bit computer? Read times. Hello, I know this is a very stupid question but how do I make a 8 bit computer? What do you recommend for a beginner in 8 bit computers?

I was Z Some go with or The only advice I would give to someone who hasn't decided is to ignore anyone who starts to try convincing you one is better than another.

z80 logisim

I'm less familiar with others. For a long time I was tempted by the and OS The first step is to pick the CPU. To do that i'd recommend finding the software that appeals. The following users thanked this post: wintech. It depends on whether you want to use a microprocessor or build it using 74 series logic gates.

Ben Eater's videos are the best I've seen for a make it yourself 8-bit computer. Just buy some breadboard and a big reel of solid-core wire. No longer care, over this forum shit Nusa Super Contributor Posts: Country:. That's a bit computer, not an 8-bit one. It was a commercial success in its day to maybe and likely predates whatever 8-bit computers you were thinking of. But it all depends on what you are looking to learn. How to make a computer using the series parts, but much of it will carry over to using the and similar.

Quote from: alanb on August 08,pm. Gaze not into the abyss, lest you become recognized as an abyss domain expert, and they expect you keep gazing into the damn thing. The following users thanked this post: I wanted a rude usernamewintech. Psi Super Contributor Posts: Country:.

Quote from: wintech on August 08,am. Greek letter 'Psi' not Pounds per Square Inch. It also has a great component API written in Java I've written a VIA component which means you can actually model your chips in software, and then build circuits around them. The following users thanked this post: granzeier. Quote from: mindcrime on August 23,am. The following users thanked this post: Kean.

Quote from: shawty on August 23,am. Quote from: granzeier on August 23,am.Your hunt for free and open source simulator software ends here. CEDAR LS is an interactive digital logic simulator to be used for teaching of logic design or testing simple digital designs. It features both low-level logic gates as well as high-level components, including registers and a Z80 microprocessor emulater. Logisim is an educational tool for designing and simulating digital logic circuits.

With its simple toolbar interface and simulation of circuits as you build them, it is simple enough to facilitate learning the most basic concepts related to logic circuits. FreeMat is a free environment for rapid engineering and scientific prototyping and data processing. FreeMat is available under the GPL license. Logic Gate Simulator is an open-source tool for experimenting with and learning about logic gates.

Hi Pork, these are free ebooks, you can download from the links itself. There is no need for images. But we will check at our end if images can be added to the article. Sign in Join.

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Sign in. Log into your account. Sign up. Password recovery. Tuesday, July 14, Advertise Contact About Magazine. Forgot your password? Get help. Create an account. Electronics For You. Home Resources Cool Stuff. Resources Cool Stuff. Among these softwares, anyone is capable for simulating analog circuits? These are all digital logic.

z80 logisim

Do you have any simulator or free software on electrical circuits o for school children. Please enter your comment! Please enter your name here.By using our site, you acknowledge that you have read and understand our Cookie PolicyPrivacy Policyand our Terms of Service.

Electrical Engineering Stack Exchange is a question and answer site for electronics and electrical engineering professionals, students, and enthusiasts. It only takes a minute to sign up. Implementing a zero flag for a register should be straight forward, it would be a logical NOR of all the bits on the register.

Something like that would work for a small number of inputs. As for a bit processor you cannot make one gigantic NOR gate with 64 inputs. The fan-in would be too high. Too many transistors would be in series.

However when it comes to the zero flag he states:. Not shown in the block diagram are the simple circuits to compute parity, test for zeroand check if a 4-bit value is less than These values are used to set the condition flags. So, although they are simple circuits I would like to know exactly how they were implemented and if they used any of the implementations proposed above or something else completely different.

There is a related question where they talk about zero flag implementation in general terms. I have got a response from Mr Shirriff himself that not only answers my question but also gives more details on the rest of the flag circuitry.

That's a good question. Carry on the other hand is much more complicated. The 4-bit operation generates the half carry, which is latched. There's a bunch of logic to handle addition vs subtraction, shifts, etc. Then the next 4-bit operation generates the fully carry. Parity is generated by exclusive-or of the first 4 bits. That result is then fed into the exclusive or of the next 4 bits to generate the final parity. It doesn't really matter so long as the answer is reliably ready in time for the next clock edge.A small 8 Bit microcontroller built with Logisim, needs 6 ticks per instruction.

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In one instruction the microcontroller can do quite a lot. All in 5 ticks! And it comes with an assembler too. It's written in Python and makes programming very easy.

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But with the update there comes one drawback: I had to increase the ticks per instruction to 6 since I got in trouble with race conditions leading to random results.

But it is still quite good though if you think of what it can do in one instruction cycle. View project log. Create an account to leave a comment. Are you sure? Buzz Pendarvis. Shashi Suman. Dylan Brophy. Become a member to follow this project and never miss any updates. About Us Contact Hackaday.

By using our website and services, you expressly agree to the placement of our performance, functionality, and advertising cookies. Learn More. Yes, delete it Cancel. You are about to report the project " Another microcontroller built in Logisim ", please tell us the reason.

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Hack a Day Menu Projects. Another microcontroller built in Logisim A small 8 Bit microcontroller built with Logisim, needs 6 ticks per instruction schuhumi. Following Follow project. Liked Like project. Become a Hackaday.Believe it or not, computers existed before microcontrollers and CPUs were around.

Building a small Z80 computer #1 - How old computers and consoles work

They used to be built using discrete parts including simple ICs and transistors. CPUs are arguably the center of modern electronics, whether it be a mobile device or a control circuit for a factory.

But how do CPUs work? What goes on inside? Then we'll build one! For example, if you wish to add two binary numbers, it is the ALU that is responsible for producing the result.

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If your program needs to execute some code if two values are equal it is the ALU that performs the comparison between the values and then sets flags if the condition is met or not. But a simple CPU say, a Z80, for example has only transistors. Computers in the past such as many of the IBM mainframe computers were actually built with discrete and series chips.

This project will be a discrete 4-bit ALU that will be constructed with series and series chips. But how do we add binary numbers that are more than one digit long? This is where the carry bit comes into play and we need to use long addition. Carry bits are used as shown below where "0 c " means "no carry bit" and "1 c " means "carry bit".

If we wish to add 10 and 10 in binary form, we would start by writing them down in the form of long addition. We add the bits up in columns using the rules above starting from the far right and moving to the left. When we have a carry from a bit addition, we move it one column to the left, where it gets included in the addition as a bit. Then we move to the next column the second from the right and add all the bits.

Z80 logisim

Notice how the carry bit is also included in this addition operation. This means we are adding three digits: 1 the carry bit1, and 0. For more information about Boolean arithmetic, check out this section of the AAC textbook. The half adder has two inputs and two outputs as shown in the diagram below. The two inputs represent two individual bits, the Sum output represents the sum of the two bits in the form of a single bit and the Carry output is the carry bit from the addition.

But what is wrong with this circuit? This circuit cannot take in a carry from a previous operation! So how do we fix this? If we use two of these adders and an OR gate, we can create a full adder that has two bit inputs, a carry in, a sum out, and a carry out.

Unfortunately, for the 4-bit ALU, it would be impractical to use discrete chips to create a 4-bit adder. So we will cheat and use a 4-bit adder IC. You can pick these up for a few dollars on eBay:.


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