Editing Sad Mac Error Codes (section)

== Macintosh 128K, 512K, 512Ke, and Plus (Short Format) ==

These original [[Macintosh Old World ROM|Old World ROM]] machines display a six-digit short code below the Sad Mac. The class code is two digits, the subcode is four digits:

{| class="wikitable"
|+ ''Class codes — Macintosh 128K, 512K, 512Ke, Plus''
! Class !! Test !! Subcode meaning
|-
| 01 || ROM checksum || Subcode is meaningless; replace the ROMs on the logic board
|-
| 02 || RAM — bus subtest || Faulty RAM chip identified by bitmask (see [[#RAM chip identification (16-bit single-SIMM bitmask)|chip table]])
|-
| 03 || RAM — byte-write subtest || Faulty RAM chip identified by bitmask
|-
| 04 || RAM — modulo-3 pattern || Faulty RAM chip identified by bitmask
|-
| 05 || RAM — address uniqueness || Faulty RAM chip identified by bitmask
|-
| 0F || Exception before OS loaded || Subcode names the [[#Common Exception Codes (Class 0F)|68000 exception]]
|}

If the Programmer's Switch (NMI button) is pressed at boot the machine will display '''0F000D''' — a Sad Mac with a "normal" NMI subcode — which is sometimes used to deliberately trigger a Sad Mac for testing the display path.

=== RAM chip identification (16-bit single-SIMM bitmask) ===

For class codes 02–05 the subcode is a bitmask of the failing data lines. Each bit corresponds to one physical RAM chip in the 128K/512K/512Ke/Plus's 16-chip RAM bank. A single failing chip lights a single bit; multiple bits set means multiple chips, an entire byte lane (e.g. {{code|0x00FF}} or {{code|0xFF00}}) usually means a failed buffer or address-line problem rather than 8 separate dead chips.

{| class="wikitable"
|+ ''Single-chip identification — original-ROM compact Macs''
! Data bit !! Board position !! Subcode bit
|-
| 0 || F5 || 0001
|-
| 1 || F6 || 0002
|-
| 2 || F7 || 0004
|-
| 3 || F8 || 0008
|-
| 4 || F9 || 0010
|-
| 5 || F10 || 0020
|-
| 6 || F11 || 0040
|-
| 7 || F12 || 0080
|-
| 8 || G5 || 0100
|-
| 9 || G6 || 0200
|-
| 10 || G7 || 0400
|-
| 11 || G8 || 0800
|-
| 12 || G9 || 1000
|-
| 13 || G10 || 2000
|-
| 14 || G11 || 4000
|-
| 15 || G12 || 8000
|}

''Note: prior revisions of this page contained two transcription errors in this table — F8 was duplicated as Data Bit 3, and Data Bit 12 was listed twice (against both G9 and G10). The corrected mapping above gives one chip per data line for all 16 bits.''

==== Worked example ====

A [[Macintosh Plus]] displays:

  '''020080'''

* Class '''02''' — RAM bus subtest failed.
* Subcode '''0080''' — bit 7 (data line D7) is bad.
* From the table, D7 lives at position '''F12''' on the logic board.

Replace or reseat the chip (or, on a Plus, the whole SIMM containing that data line — the Plus uses 30-pin SIMMs rather than soldered chips, so the F5–G12 lettering corresponds to the chip locations on the SIMM itself).