Circuitry Mac OS
Low End Mac'sOnline Tech Journal
Dan Knight - 2001.05.29
This article is adapted from a series of articles andsidebars in the February 1984 issue of Byte magazine. Although some of thedetails included in this article are specific to the original Mac, manyalso apply to other compact Macs, such as the Plus and SE/30,
The Macintosh wasintroduced on January 24, 1984, so the staff of Byte was working with late prototypes orearly production units when they wrote these articles, which include along interview with the Macintosh development team.
The goal of the Macintosh project was to bring the concepts andtechnology of the $10,000 Lisa to a much broader audience,mostly by figuring out how to adapt it to a far less costly computer.The original Macintosh was released at $2,500, a fraction of the Lisa'soriginal cost. Steve Jobs said this fulfilled the design goal to 'builda really cheap implementation of Lisa's technology....'
Automated schematic rendering Unlike other circuit design software tools, where you need to spend time 'neatly' placing your wires and selecting colors for each wire, Cirkit Studio intelligently and automatically draws wires for you. Each wire is assigned a unique color automatically for better visualization. Mac OS: Computer: Mac, with an Intel processor; Memory: 1 GB minimum of installed RAM, 2 GB recommended; Mac OS version: 10.6.4 or later; Connection and power: Any available USB port on a Mac; Software: GarageBand, Logic Pro, MainStage, Pro Tools (v. Requires Mac OS X 10.4 or higher, runs on PowerPC and Intel-based Macs. Oh and by the way, the EdLibPlay package also includes a collection of nearly all available D00 music files for your listening pleasure. With proper filenames and headers. Oh c'mon guys, no need to be so effing modest! Whatever reasons @Igor had for mentioning the users he did, I see a commonality: comment frequency. Is not hard to pick out those who post circuits or comment on others' more often, helpful or not.
Bill Atkinson summed up that part of the design: 'We want to mostcomputer that you can get for the least dollars so that the most peoplecan have it....' Maybe that's the origin of 'The computer for the rest of us,' although theMac has never really sold for 'the least dollars.'
Memory
The biggest compromise was memory - Lisa came with 1 MB andcould be expanded to 2, while the Macintosh had 128 KB of memory.(Apple hoped to expand that to 512 KB by the end of 1984; they had towait for 256 kilobit memory chips. Once they became available, Applereleased the Macintosh512K in September 1984.) The operating system and programs had tobe completely recreated for the low-RAM footprint of the Mac, since theLisa OS alone required more RAM than the Mac had.
Making life even more difficult, just over 21 KB of that precious128 KB of memory was used for video, leaving under 107 KB available forthe operating system and applications. (That memory used for video is abig part of the reason Apple didn't produce a 64 KB Macintosh - videowould have used one-third of its memory.)
Perhaps the first thing to go was multitasking; with so little RAM,the Mac couldn't be expected to run multiple programs at the same time,although Apple did provide for Desk Accessories, applets that could runat the same time as full-fledged applications.
Another clever memory-saving, speed-boosting move was switching fromcompiled Pascal to 68000 machine language for the operating system.
A third way of getting around the limitation of 128 KB of RAM (dueto high memory prices, from what I've heard, since even the originalMac OS really cried out for 256 KB of RAM) was building 480handcrafted, optimized toolbox routines into ROM, where the OS and allMac applications could access them. Not only did this reduce the needfor RAM, since these routines were run directly from ROM, but it alsohelped enforce a standard user interface, since all programmers coulduse these routines.
Speed & Video
Although we call this an 8 MHz computer, nit-pickers are quickto point out that the 8 MHz processor 'only' ran at 7.83 MHz. (Why theyget so bent out of shape over this 2% difference is beyond me.)Compared with the 5 MHz Lisa, the CPU in the Macintosh was a speeddemon. And compared with the 4.77 MHz 8088 processor common in DOSmachines, the Mac's 68000 was both faster and more efficient, since itaccessed memory 16 bits at a time vs. the 8-bit architecture of the IBMPC.
Working to squeeze the most out of every component, ROM and RAM ranat full speed on the system bus, so there was no speed penalty usingthe toolbox routines in ROM. However, because RAM was triple-ported(that is, it was used by the CPU, for screen display, and for sound),the 68000 processor had access to RAM only part of the time.
Video and sound are time-critical; they had to run at fixedintervals. In simple terms, that means that when the Mac needed to drawon the screen or make a sound, the CPU was temporarily shut out fromaccess to RAM.
According to 'Macintosh System Architecture' by Burrell C. Smith,this was handled in a very clever way where the CPU and videoalternated access to memory:
Each 512-pixel horizontal line consists of 32 words[16 bits or 2 bytes] of data . . . followed by 12 words ofhorizontal blanking. The last memory bus cycle of each horizontal lineis reserved for sound DMA . . . The update rate of the soundchannel is equal to the horizontal video rate, or 22,254.55 Hz.
Access to RAM is divided into synchronous time slots,with the 68000 and video circuits sharing alternate word accessesduring the love portion of the horizontal video display line. . . Although access to RAM is divided three ways, the68000's share is maximized by giving it access to unused cycles duringhorizontal and vertical blanking. This way, 68000 access to RAMaverages to a speed of about 6 MHz.
The Mac only displayed black and white pixels - no grays - and had arelatively small screen of 512 x 342 pixels, which also helped thedisplay run quickly. (The video was also internally clocked to 60.15Hz, making it possible to run the Mac on 50 Hz current without messingup the display.)
Even the fact that the Mac uses square pixels, something we take forgranted today, greatly simplified the display routines and helped makefor faster screen displays.
Beyond 8 MHz
Because of the video architecture, moving to a 16 MHz CPU, as withthe SE/30 or Brainstorm upgrade for the Mac Plus, meantthat the CPU didn't share time 50:50 with the display, but instead usedthree cycles for every one needed by video. Because of this, 16 MHzcompact Macs could run over twice as fast as 8 MHz ones.
This is also a reason that Apple never released a grayscale compactMac - 8-bit video would require eight times as much data going to thedisplay, which would mean 8 video accesses for every CPU access tomemory on an 8 MHz Mac (4:1 on a 16 MHz Mac), which would have slowedthe computer to a crawl. Color and grayscale video had to wait fordedicated video cards (on the MacII and SE) and faster CPUswith 32-bit data paths (the IIci).
Reliability and Simplicity
Mac prototype with Twiggy drive.
The Mac had a fraction as many chips as the Lisa and used only twocircuit boards, one for analog circuitry (power) and on for digital(the motherboard). Decreasing the number of parts not only helpedreduce costs, but also meant greater reliability. Reliability was alsoone of the reasons Apple moved from 5.25' floppies to Sony's new 3.5'floppy. (As the photo to the right shows, Apple built some Macprototypes using the Twiggy drive from the Lisa - look at how wide thefloppy slot is.)
One reason Apple was able to squeeze 400 KB onto a 3.5' floppy wasSteve Wozniak's clever disk interface work done for the Apple II. Bycreating a high-speed version of the Woz design, Apple had 'twice themargin of MFM' (the standard way of encoding data) and was able tostore 400 KB on a single-sided floppy that other vendors used at 270 KBcapacity.
Another clever hack was running the floppy at different speedsdepending on how close the track was to the outer edge, since there wasmuch more room to store data there. This also contributed to the highercapacity - and was a key reason other computers would have a difficulttime reading Mac floppies.
The 3.5' Sony drive was customized for Apple to include 80 tracks(earlier drives had only 70 tracks) and support rotation speeds of390-600 rpm. This allowed the Mac to store data at a constant lineardensity under control of the IWM (integrated Woz machine) chip. Thedrive had 400 possible speeds, and the IWM chose the proper speed bymonitoring data read speed and attempting to maintain 489.6 Kbps.
If all that sounds complex, remember that the whole thing iscontrolled by the IWM, while IBM's floppy controller needed 45-50 chipsto run a drive with a single rotation rate. Apple's elegant solutionwas simpler and more reliable - and definitely a sign of a company thatthinks different.
To this day, Macs have a reputation for greater reliability thanother personal computers.
Hardware
Bucking conventional wisdom, Apple adopted a single-button mousebecause their usability tests found that new users were often unsurewhich button to use on multi-button mice. Apple has never produced amulti-button mouse.
The two serial ports were not the typical RS232C ports found onother computers, but RS422A ports, which not only supported RS232functionality, but allowed Apple to develop LocalTalk, an inexpensive,easy, relatively fast (230.4 kbps) networking protocol that wasstandard on all Macs until the iMac. With an externalclock, the serial port could support 1 Mbps, a feature which somecompanies would later use to accelerate LocalTalk.
In addition to the mouse and serial ports, the only other data porton the Mac was for a second floppy, although this would later beadapted for serial hard drives as well.
These serial ports remained in use until Apple adopted USB with theiMac in 1998.
The keyboard connected to the front of the Mac using a coiled cablethat looked like it came from a telephone handset, although it was notinterchangeable with telephone cables. The keyboard itself had no arrowkeys (you were expected to use the mouse), no numeric keypad, and nofunction keys. It did have shift, option, and command keys, but noescape or delete keys.
Apple added arrow keys to the keyboard with the Mac Plus, andincluded function keys on the Extended Keyboard for the Mac II andSE.
The Physical Design
An early design goal was to make the system compact and portable,but early designs had a horizontal layout with the floppy drive next tothe screen - just like the Osborne, Compaq, and most other portables ofthat era.
One day Steve Jobs 'said that we didn't want portability to be theprimary aspect of this, but we did want it to take minimum desk space.'Putting the floppy below the display solved that and also put themonitor at a better working height.
Not only did Apple begin the Macintosh with this kind of design, butthis also forms the foundation of the iMac's design.
The Operating System
One of Apple's clever hacks in adapting the Lisa OS to a one programat a time environment was retaining the Lisa's clipboard, which let theMac user copy a piece of text or a graphic from one program, quit thatapplication, launch another, and paste the item into the secondapplication. We may expect that today, but with the Lisa and Mac, itwas an innovative feature.
Another key issue for both the Lisa and Mac operating systems wasfull integration of graphics and mouse with the operating system. Thisis in contrast to operating systems such as the consumer version ofWindows (a graphical shell running on top of DOS) and Mac OS X (Aquarunning on top of Unix). While it is possible to break out of the GUI(Graphical User Interface) into a programmers mode, even that usestoolbox routines to display text; you cannot use the Mac OS withoutaccessing graphics.
Desk accessories were Apple's solution to the occasional need to runmore than one application at a time. These applets were specificallydesigned to run at the same time as other applications to display thetime, manage a scrapbook, etc.
The Mac OS responds to peripherals on an asynchronous basis, whichpermits the computer to work with more than one peripheral at a time.For example, this means the user can move the mouse while accessing afloppy disk.
Peripherals
The Mac's two peripherals were the ImageWriter printer and anexternal floppy drive. The $595 ImageWriter could easily (albeitslowly) print in graphics mode to emulate what was on the screen. The$495 external floppy was a practical necessity, since it meant youcould switch programs or save files on a work disk without ejecting thesystem/application disk.
It was two years before Apple produced the HD20, a hard drive forthe Mac that connected via the slow floppy port. Shortly thereafterthey released the Mac Plus,the first Mac with a SCSI port, which made it easy to add faster harddrives. Apple still offers SCSI as a build-to-order option for thePower Mac G4.
Trivia
McIntosh is the apple, but Macintosh is the Apple computer - andalso an overcoat in the UK. The original code name, McIntosh, wasmisspelled, and Apple decided to stick with it.
MacWrite was developed under the name MacAuthor, and MacDraw wasgoing to be called Mackelangelo. Thank goodness they changed them.
The Macintosh was designed for an international market; the onlytext on the rear panel was Apple. Everything else was marked withicons.
All the early Macs (all models prior to 1987) were fanless, justlike current iMacs and theCube.
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I recently discovered iCircuit, a neat little circuit simulator app that works on Mac as well as mobile devices. It’s perfect for simple effects pedal designs.
A common way to prototype simple electronic circuits such as used in guitar effects pedals is to use a breadboard and real components. This is great for audio projects because you get real results that you can actually hear right away.
Breadboarding, however, it has it’s challenges:
The top four breadboarders headaches:
1 – You have to buy all the parts.
2 – Anything complicated quickly turns into a rats nest.
3 – Routing sensitive signal lines is a nightmare.
4 – It’s hard to get high pin density SMD micro controllers onto a breadboard.
Now admittedly the last two may not be an issue for basic effects pedals, but we are seeing more and more complex micro controllers and CPU’s becoming available to the maker community, and it’s only a matter of time before home-brew digital guitar effects come more commonplace.
Circuitry Mac Os X
An alternative to breadboarding is to use a computer based circuit simulation application. The ones most commonly used in professional electronics are based on SPICE. Many companies that develop integrated circuits create their own SPICE based tools, and Linear Devices makes theirs, LTSPICE available for free which is pretty cool.
The problem for Mac users is that although there is a free LTSPICE version for OS X, the user interface is pretty clunky. It lacks a tool bar for the schematic editor which makes editing layouts directly pretty cumbersome and you have to import schematics in from something else which is a whole other business. The Windows interface is much nicer.
Circuitry Mac Os Download
So this lead me to search for a simulator for OS X and hence to iCircuit. It’s a neat little app that’s great for students and hobbyists and supports both analog and digital circuits with inbuilt oscilloscope and meter simulations. There are a couple of headline features:
1. OS X, and IOS versions for Mac, iPhone and iPad
2. Real time simulation
The OS X version works fine and has a nice Mac like user interface. I didn’t try the iOS version yet but the screenshots look very nice.
The realtime simulation is quite fun. Most simulators provide simulation results in the form of a series of reports, but iCircuit simulations run as you work, so you can quickly see how changes to components impact your project, and you can monitor signals with the scope and meters as you go.
The library of components in iCircuit is pretty limited, so it’s not going to replace a SPICE type tool for professional simulation, but for learning and testing simple circuits it’s pretty useful and way more fun.
There are links to download iCircuit from the developers website. At time of writing, there are versions for OS X, Windows, iOS, and Android at between $4.99 and $9.99 depending on platform.