Editing Sad Mac Error Codes (section)

== Understanding Sad Mac Error Codes ==

A Sad Mac is the visual half of a ROM-level startup test failure. On every machine it has the same structural meaning: the boot code in ROM runs a fixed sequence of tests on the CPU, RAM, support chips, and (on later models) the Power Manager and VRAM. If any test fails, or if an unexpected 68000-family exception occurs before the operating system has loaded enough to display a bomb dialog, the ROM jumps to a small routine that draws the Sad Mac and prints the contents of CPU registers '''D6''' (minor / specifics) and '''D7''' (major / which test, plus flags).

Two formats exist:

* '''Short format''' — used on the [[Macintosh 128K|128K]], [[Macintosh 512K|512K]], [[Macintosh 512Ke|512Ke]] and [[Macintosh Plus|Plus]]. A single six-digit line, '''YYXXXX''', where '''YY''' is the two-digit class code and '''XXXX''' is the four-digit subcode.
* '''Expanded format''' — used from the [[Macintosh SE|SE]] onwards (SE, SE/30, Classic, Classic II, Color Classic, LC family, Performa, Mac II family, Centris, Quadra, and the NuBus Power Macs). Two lines of eight hex digits each, displayed as:
  : Line 1 (D7) — '''FFFFTTEE''' where ''FFFF'' = test-manager flags, ''TT'' = test number, ''EE'' = exception number (ORed in if an unexpected exception interrupted a test).
  : Line 2 (D6) — minor error / additional information (failing bit mask, address-line XOR, stack pointer, etc.).

When an exception occurs ''during'' a test the exception number is logically ORed into the lower word of the major code. So if an address error (0200) hits during the [[#RAM Test on Mac II|RAM Bank A test (test 03)]], the major code reads '''0203''', not '''0200''' or '''0003''' alone — you read it as "address error while running test 3". This OR-encoding is the single most important thing to know about decoding expanded codes; without it most published code tables look contradictory.

=== Test Manager flags (upper word of D7) ===

On expanded-format machines the upper four hex digits of line 1 are not random — they are the live state of the Test Manager. Each bit has a meaning:

{| class="wikitable"
|+ Test Manager flag bits (D7 bits 16–31)
! Bit !! Flag !! Meaning when set
|-
| 16 || Message queued || A diagnostic message is pending in the serial output queue
|-
| 17 || SCC INITed || The SCC serial controller has been initialised
|-
| 18 || No-sleep || Auto-sleep is inhibited (Portable / PowerBook)
|-
| 19 || Star received || A {{code|*}} command character has been received over the serial diagnostic port — Test Manager is active
|-
| 20 || ASCII input || Test Manager expects ASCII command input (clear = hexadecimal)
|-
| 21 || Echo || Echo received characters back over the serial port
|-
| 22 || Timer || Timer-based testing enabled
|-
| 23 || Newline || Emit CR/LF after every Test Manager response
|-
| 24 || Unexpected exception || An unexpected exception was caught — exception code is ORed into the test number
|-
| 25 || Programmer's key || The Programmer's Switch (NMI button) was pressed
|-
| 26 || VIA Test || A VIA test is currently asserting itself on the bus
|-
| 27 || Bus errors OK || Bus errors are expected (used during chip-probe tests)
|-
| 28 || Stop on error || Halt on the first error rather than continuing
|-
| 29 || Loop on error || Loop the failing test indefinitely (for scope/logic-analyser work)
|-
| 30 || Store in PRAM || Persist test results to PRAM
|-
| 31 || Boot after test || Continue booting after diagnostics complete
|}

In practice, when looking at a Sad Mac code in the wild, the upper word usually reads '''0000''' (no flags set, normal boot), '''0001''' (a serial message had been queued before the failure), or a value with bit 24 (0x0100) and/or bit 25 (0x0200) set if the failure was exception-driven or operator-induced.