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Macintosh 128K

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Macintosh 128K
Macintosh 128K
Specifications
ManufacturerApple Computer, Inc.
TypePersonal Computer
ReleasedJanuary 24, 1984
DiscontinuedOctober 1, 1985
Intro priceUS$2,495
CPUMotorola 68000 @ 7.83 MHz
Memory128 KB RAM (not expandable)
Storage400 KB 3.5" floppy drive
Display9" monochrome CRT (512×342 pixels)
SoundMonaural 8-bit, 22 kHz (4-voice)
Dimensions13.6" H × 9.6" W × 10.9" D
Weight16.5 lbs (7.5 kg)
OS / FirmwareSystem 1.0 – System 3.2
PredecessorNone
SuccessorMacintosh 512K
CodenameAnnie, Mac, Twiggy Mac
Model no.M0001

The Macintosh 128K was Apple's first mass-market personal computer to feature a graphical user interface and mouse, released on January 24, 1984.[1][2] Designed to be an affordable and user-friendly computer, the Macintosh 128K was revolutionary in its time but was severely limited by its 128 KB of RAM, which was soldered to the logic board and could not be expanded.[3] This memory constraint became the system's defining limitation, forcing users into constant "disk swapping" when running applications and preventing the use of more demanding software.[4]

The system used a Motorola 68000 processor running at 7.83 MHz and introduced 3.5-inch floppy drives to the personal computer market.[5][6] The Macintosh 128K originally shipped with System 1.0, running the bundled MacPaint and MacWrite applications, and could support up to System 3.2.[7] Due to severe memory constraints, applications like MacDraw, Microsoft Excel, and PageMaker required the later Macintosh 512K or Macintosh Plus models. The 128K's limited memory meant that even switching between MacPaint and MacWrite required ejecting and inserting floppy disks multiple times—earning it the nickname "the $2,500 disk swapper" among early users.

General Maintenance

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For detailed general maintenance practices including cleaning, PRAM battery handling (not applicable to 128K), connector corrosion prevention, and voltage adjustments, please see the dedicated Macintosh 128K/512K General Maintenance page.

PCB Schematics & Service Manual

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The Macintosh 128K features two primary PCBs: the Logic Board and Analog Board. Below are the detailed schematics and the official Apple service manual. Due to the similarities with the Macintosh 512K, the logic board and analog board are nearly identical, with the 512K using additional RAM chips. The Macintosh Plus also shares the same analog board design (with the addition of a PRAM battery socket) as the 128K and 512K.

Apple Service Manual

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The Apple Service Manuals can be found on the Apple Service Source page.

Logic Board & Analog Board Schematics

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Macintosh 128K PCB Schematics
Logic Board Analog Board
Macintosh 128K Logic Board
Macintosh 128K Logic Board Schematic
Macintosh 128K Analog Board
Macintosh 128K Analog Board Schematic

Capacitor Replacement Guide

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Detailed capacitor replacement guidelines for both analog and logic boards can be found on the Macintosh 128K/512K Capacitor Replacement Guide page.

Retrobrite

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Instructions on addressing case yellowing through Retrobrite treatment can be found on the Retrobrite page.

Troubleshooting

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A comprehensive troubleshooting guide addressing no power, chime but no video, intermittent operation, Sad Mac errors, and floppy drive problems is available on the Macintosh 128K/512K Troubleshooting page.

Technical Details

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System Architecture at a Glance

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Sub-system Specification (Macintosh 128K, Jan 1984)
CPU Motorola 68000 @ 7.8336 MHz (15.667 MHz ÷ 2)

Design Philosophy Behind Motorola's MC68000– Thomas W. Starnes, BYTE Magazine, April 1983—link(accessed 2025-01-25)


Bus width 16-bit data • 24-bit address (16 MB logical space)
ROM 64 KB "Boot/Toolbox" — 4 × TMS2532 16 KB mask ROMs

Macintosh 128K: Technical Specifications, Apple Support—link(accessed 2025-01-25)


RAM 128 KB DRAM (64 × MT4264-15, 150 ns) — non-expandable

Apple Macintosh Microcomputer, National Museum of American History—link(accessed 2025-01-25)


Video 512 × 342 mono — 22.1 KB frame buffer shared with system RAM
Display DMA @ 15.667 MHz steals ~60% of CPU cycles during active scan
Sound 8-bit DAC via VIA Port A • 22.254 kHz PWM → RC filter → speaker
Storage Sony OA-D34V 400 KB GCR 3.5″ drive • IWM controller
Variable speed: 394 RPM (outer) → 590 RPM (inner tracks)
Ports DE-9 mouse • DE-9 keyboard • DB-19 ext. floppy • RS-422 serial × 2 • 3.5mm audio
Expansion None — no slots, non-upgradeable RAM

Memory Map (Physical)

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Macintosh 128K Address Space
Range Size Purpose
$000000 – $01FFFF 128 KB DRAM (actual system memory)
$020000 – $03FFFF 128 KB DRAM mirrors (unused on 128K)
$400000 – $40FFFF 64 KB ROM (Boot/Toolbox)
$410000 – $41FFFF 64 KB ROM mirrors
$600000 – $6007FF 2 KB VIA 1 (system timers, sound)
$800000 – $8007FF 2 KB VIA 2 (keyboard, mouse, floppy)
$A00000 – $A007FF 2 KB SCC (serial communications)
$C00000 – $C007FF 2 KB IWM (floppy disk controller)
$E00000 – $EFFFFF 1 MB Slot space (unused on 128K)
  • Frame-buffer:* 22,016-byte display buffer at $12700 – $1FFFF (top of RAM); DMA fetches pixels during active scan, reducing CPU performance by ~40%.

ROM Layout (64 KB)

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Offset Size Module
$400000 4 KB 68000 exception vectors + Boot code
$401000 16 KB QuickDraw 1.0 (monochrome bitmap routines)
$405000 24 KB Macintosh Toolbox (Window/Menu/Dialog Managers)
$40B000 8 KB SANE (Standard Apple Numeric Environment)
$40D000 6 KB Sony IWM floppy driver
$40E800 6 KB System utilities & resource manager

Video Timing

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Parameter Value
Pixel clock 15.667 MHz ÷ 2 = 7.833 MHz
Horizontal total 704 pixels (512 active)
Vertical total 370 lines (342 active)
Refresh rate 60.15 Hz
Memory bandwidth ~22 MB/s (shared with CPU)

Floppy Disk Format (400 KB Sony GCR)

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  • 80 tracks • single-sided • variable-speed GCR encoding
  • 12 sectors/track (outer) decreasing to 8 sectors/track (inner)
  • Sector size: 512 bytes • Total capacity: 409,600 bytes
  • IWM controller with software-controlled stepper motor timing

Key I/O ICs

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  • 6522 VIA × 2 (MOS Technology) — system timers, sound PWM, I/O handshake[8]
  • 8530 SCC (Zilog) — dual RS-422 serial ports (230.4 kbps max)
  • IWM (Apple custom) — Integrated Wozniak Machine floppy controller
  • MC6883 PAL (Motorola) — address decode, 60 Hz VBL interrupt generation

Sound System

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68000 writes 8-bit samples to VIA Port A at 22.254 kHz. VIA Timer 1 toggles Port B to create PWM signal, which passes through RC filter (R=10kΩ, C=0.1µF) to internal 8-ohm speaker or rear 3.5mm audio jack.

Memory Limitations & Performance Impact

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The 128K's memory constraint created several operational challenges:

  • Application switching required complete disk swaps due to insufficient RAM
  • MultiFinder (cooperative multitasking) was impossible until 512K model
  • Virtual memory concepts were unused due to single-floppy, no-HDD configuration
  • System overhead consumed ~20KB, leaving only ~100KB for applications and documents

Hardware/Easter Facts

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  • First consumer computer to ship with 3.5" Sony floppy drives (later industry standard)

Insanely Great: The Life and Times of Macintosh, the Computer that Changed Everything, Penguin Books

ISBN 0140291773}


  • Hidden startup chime: Hold ⌘ + Option + Shift + Delete during boot for deep bass tone

Revolution in the Valley: The Insanely Great Story of How the Mac Was Made, O'Reilly Media

ISBN 0596007191}


  • Logic board manufactured by Apple in Fremont, California facility

1984 Timeline of Computer History, Computer History Museum—link(accessed 2025-01-25)


  • Case design influenced by Braun radio aesthetics via Hartmut Esslinger
  • "Twiggy Mac" prototype used dual 5.25" drives before Sony partnership

Original Apple Macintosh 128K Brochure 1984, Apple Computer—link(accessed 2025-01-25)


  • Custom Motorola 68000 processors marked "SC87839L" used in development prototypes

Design Philosophy Behind Motorola's MC68000– Thomas W. Starnes, BYTE Magazine, April 1983—link(accessed 2025-01-25)


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  • Macintosh 128K system
    Macintosh 128K system
  • Logic board (64 × 64Kbit DRAM chips visible)
    Logic board (64 × 64Kbit DRAM chips visible)
  • Analog board (shared with 512K/Plus)
    Analog board (shared with 512K/Plus)
  • System 2 Desktop
    System 2 Desktop
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References

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  1. ↑ 1984 Timeline of Computer History, Computer History Museum—link(accessed 2025-01-25)
  2. ↑ Apple Macintosh Microcomputer, National Museum of American History—link(accessed 2025-01-25)
  3. ↑ Macintosh 128K: Technical Specifications, Apple Support—link(accessed 2025-01-25)
  4. ↑ Revolution in the Valley: The Insanely Great Story of How the Mac Was Made, O'Reilly Media
    ISBN 0596007191}
  5. ↑ Design Philosophy Behind Motorola's MC68000– Thomas W. Starnes, BYTE Magazine, April 1983—link(accessed 2025-01-25)
  6. ↑ Insanely Great: The Life and Times of Macintosh, the Computer that Changed Everything, Penguin Books
    ISBN 0140291773}
  7. ↑ 1984 Macintosh Manual, Apple Computer, Inc.—link(accessed 2025-01-25)
  8. ↑ Design Philosophy Behind Motorola's MC68000 (Part 2)– Thomas W. Starnes, BYTE Magazine, May 1983—link(accessed 2025-01-25)
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