MicroprocessorTasmSoftwareX86 assembly language is a family of backwardcompatible assembly languages, which provide some level of compatibility all the way back to the Intel 8008 introduced. X86 is a family of backwardcompatible instruction set architectures based on the Intel 8086 CPU and its Intel 8088 variant. The 8086 was introduced in 1978 as a. Helm PCB 8. 08. Computer. Published on 2. November, 2. Note. Unfortunately, this article is not complete and has been so for several years. However, I hope you can still get some use out of whats there. MayDR_FGmSQ/V8bxoNm-KVI/AAAAAAAABQM/jK8g3ZBYf40M3-d4dv0acI47f9H-rooWgCLcB/s1600/Screenshot_26.jpg' alt='8086 Microprocessor Tasm Software' title='8086 Microprocessor Tasm Software' />The project. This project aims to develop an Intel 8. SBC and to add various different peripherals to it. The project started out when someone was selling some 8. Bay, and so just out of interest, I bought them and then started to search online to see what I could do with them. From these searches, I found out a book called Build your own Computer based on the 8. Walter Fuller ISBN  0. This book got me started, but I also found another book The 8. Project Book by Robert Grossblatt ISBN 0. In my opinion, this second book is better because the author explains a lot more about whats going on whereas the first book was meant as a lab supplement for a college course, and so doesnt go in depth. Regardless, theyre both good books and probably worth purchasing if you plan on doing this project yourself. When I set out to make this SBC the aim I had in mind was to build a basic SBC but have the functionality to expand it by plugging other boards into it. Why would I want to build something using an old 8. Well, Ive found that designing and building a microprocessor system is quite different from building a microcontroller based circuit. This is because with a microprocessor you have complete access to the chips address, data and control lines and you add the systems RAM, ROM and IO yourself, meaning you have complete control on the specifications of your system, whereas with a microcontroller, youre pretty much limited to what youve bought. Also, its fun Where to buy the components The Intel 8. July 1. Id still be able to buy the 8. Well, to my surprise, I found that I could get most of the support chips from Farnell, and I also found that Intersil Semiconductor are still making and selling the 8. I dont think theres any worries there. General outline. The very basic microprocessor circuit is going to consist of the following clock, processor, ROM, RAM and the ability to read and write from IO and so thats what I aimed for first. First things first the clock. Clock circuit. All microprocessors need a clock source in order to run and for the 8. This chip supplies the 8. RESET and READY. The RESET signal does what it says on the tin and resets the 8. This line can also be used by other peripherals on the computer so that they reset when the 8. The READY line is useful if you have some slow IO interfacing to the processor. If your IO needs to some extra time to deal with the processors request, it can take the READY line low. The processor will the wait for READY to go high again before resuming. Instead of connecting your peripheral to the 8. READY line directly, the 8. READY lines for your IO RDY1 and RDY2 which are synchronised the the system clock, and so youd connect your IO READY signal to one of them. Pins AEN1, AEN2 and ASYNC control the operation of RDY1 and RDY2. However, since the 8. READY signals, so AEN1 and AEN2 are tied low and ASYNC is left floating. The 8. 28. 4 can take its clock source from either a crystal, or an oscillator, and you tell it which one via the FC pin. If using an oscillator, the output of the oscillator is connected to EFI and FC is tied high. If youre using crystal like me, then the crystal is connected across X1 and X2 and FC is tied low. The oscillator is divided by 3 and given a 3. CLK. Because the 8. I have runs at 5. MHz, I have used a 1. MHz crystal. The 8. PCLK. This clock is at half the frequency of CLK and has a 5. The 8. 28. 4 also provides a buffered output of the full oscillator frequency on the OSC pin. The RES pin is an active low signal. Bringing this pin low tells the 8. Ive used a simple resistor and capacitor circuit to hold the system in reset for a short while after you switch on the power, to give time for the power supply and anything else in the circuit to stabilise. When the power is first switched on, the capacitor is discharged and so RES 0. V and the system is held in reset until the capacitor charges up and then the system is allowed to run. In my circuit, Ive used a 1. F capacitor, giving a power on reset time of 4. The button then allows you to reset the system at any time. The circuit diagram is shown below Testing the clock circuit. Testing the 8. 28. With no chip and the power on, first check that the reset circuit works as expected. When the power is turned on, the output of the reset circuit should rise to 5. V then stay there. Pressing the reset button should bring it down to 0. V and when the button is released, the output should go back to 5. V. Once thats done, plug in the chip and apply power then take a scope and check out the output pins. The RESET line should normally be low until the button is pressed. It should then remain high until the button is released, where it goes back low. The following photo shows my scope output of the RESET and RES lines NOTE Im using 1. Vdiv 1. 00. 55 and so in actual fact its 5. VdivNext check the PCLK and CLK outputs. The CLK output should be a 5. MHz square wave with 3. PCLK output should be a 2. MHz square wave with a 5. The following photo shows my scope output of CLK and PCLK That wraps it up for the 8. CLK, RESET and READY lines will be connected to the 8. Adding the 8. 08. Processor. Now that the clock circuit is up and running, we need to prepare the 8. This stage involves connecting the 8. The 8. 28. 4s READY, RESET and CLK outputs should go to the 8. READY, RESET and CLK inputs. Next, the following pins should be tied to ground NMI Non maskable interrupt. How To Install North Air Windows here. Taking this pin high will force the 8. The reason why this is called a non maskable interrupt is because unlike all the other interrupts on the 8. For now, this pin is tied low until we play around with interrupts. INTR Interrupt request. This pin is taken high if a device wants to trigger a particular interrupt on the 8. For now, this is tied low. TEST This pin is used as a debug tool. Basically, the 8. WAIT. When the 8. TEST. If TEST is high, then the 8. TEST goes low. In effect, TEST and WAIT can be used to add breakpoints in your code you can pause execution at a particular point, then maybe have a snoop around the circuitry and see that everything is working as expected. HOLD This pin is used if another part of the circuit needs to take control over the address and data buses and control lines. If HOLD is taken high, the 8. For this SBC, I dont plan to use this, so its tied low. GND1, GND2 The 8. The following 2 pins are tied high MNMX MinimumMaximum mode. The 8. 08. 8 is designed to operate as a standalone processor or as one of a multiprocessing team. If MNMX is tied high, the 8. If MNMX is pulled low, the 8. MNMX changes how some of the control lines behave. This SBC is going to have just one processor and anyway, its simpler to work in minimum mode, so MNMX is tied high. Vcc Without power, the 8. Below is a circuit diagram of the setup so far Theres nothing much to test here, apart from checking your wiring and making sure the correct voltage levels and signals are present on the pins connected. Memory. As mentioned previously, one of the things which makes microprocessors so flexible is that you have complete control over how much memory you add and how its mapped. You can have as little or as much as you want within limit of course and arrange it how you see fit.