Olivetti M21/M24 (AT&T 6300)
The M24 went to the US as the AT&T 6300. It had an 8086, so was faster than the PC, albeit difficult to work on.
POST codes are sent to 378 (LPT1). If a fatal error occurs, it performs more initialization of DMA and interrupt controller circuits, tries to display an error message, complements the lower 6 bits of the POST code, sends the result to port 378, and halts the CPU, so numbers will flicker on the POST display with bit 6 on and the lower bits running from 0 upward. The codes start at 40 because a black box was used to monitor POST status at the parallel port. Bit 6 was set true (to a 1) to alert the box that the POST was starting.
Code Meaning

40 CPU flags and register test failed (fatal)

41 BIOS ROM checksum test failed (fatal)

42 Disable pdma controller command and test 8253 timer channel 1, mode 2, refresh counter (fatal); display sub-error code of 1 if interval is below window, 2 if above, and 3 if timer does not respond.

43 8237 DMA controller test failed (fatal)?amaster clear the controller, set the mask register, read the control registers, test all 8 read/writeable channel registers. Test registers 0-3 DMA address and count with FFFF then 0000.Set up channel 0 for 64K RAM address refresh. Set up memory-to-I/O transfer, unmask the RAM refresh, and let refresh begin for the first time. Set up the 8253 for proper refresh count. Test for unexpected DMA request (suberror 3), and init DMA channel 1 (not used), 2 (floppy), 3 (display), and init nibble latches. Check for proper DMA transfer into lowest 64K bank of RAM (suberror 4 if parity error).

44 8259 PIC test failed (halt)?ainitialize stack to lower 64K RAM area just tested, init and disable 8259A, set up interrupt vectors in RAM, set up software then hardware diagnostic interrupt vectors, test software interrupts, then hardware interrupts. Disable interrupts via 8259 mask register, check for hot interrupts, convert hot mask to IRQ number, save any error code, install interrupt vectors, initialize video, and display any error messages (H:#, where # is the hot IRQ#).

45 Install real interrupt vectors, determine system configuration from switches, and initialize video mono and color. Set video mode 3, clear the screen, and display any passing error messages for CPU, ROM, DMA, or interrupt controller. Size and clear RAM at every 64K bank past the lowest 64K, displaying the tested RAM as test progresses. Display any error in form cc:y000:zzz:wwww:rrrr, where cc is the config number, y the failing segment, z the offset, w the written data and r the read data. Test the MM58174 clock calendar, and display message if fails Test 8253 real time clock count capability, and tone generator. Display any error, and halt if failure.

48 Send beep to display and initialize all basic hardware. Init 8041 keyboard controller, determine parallel port configurations and test their registers, determine serial 8250 and Z8530 configurations, check for game card, set up interrupt controller, set all 4 Z8530 serial controllers to 9600 baud, no parity, 1 stop and 8 data. Set up interrupt vectors, initialize RAM variables, clear the screen, initialize the hard disk controller, test for and initialize option ROMs, verify ROM checksums okay, initialize floppy disk controller, allow user to select alternate Z8000 processor if installed and perform INT 19 cold boot.