Infoportal Olivetti M24 and Olivetti M24SP

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The M24 was sold by Olivetti from 1983. No computer at the time was more compatible with the IBM PC than this one, and it was significantly more powerful. AT&T, which owned 25% of Olivetti, sold the device in America as the "PC 6300." Xerox also marketed it as the "Model 6060." Apart from minor design differences on the outside, there are no differences internally compared to the original Olivetti-branded version.

The M24 was so successful internationally that Microsoft supported it as early as Windows 1.0, under the "AT&T PC 6300" branding.
In Austria, the M24 (without the SP) with 640KB of RAM cost a hefty ATS 125,900,- (source: Page 32 ) - equivalent to an astonishing € 24,000,- (according to eurologisch.at ) in 2023! However, it was still cheaper than the original IBM PC-XT, which cost the same amount but only offered 4.77MHz and CGA.

Overview

Technical Specifications:

• Manufactured between: 1983 - 1989
  
• Retail Price (Austria 2020, inflation-adjusted): With 8086 8 MHz, color monitor, 640 KB RAM, and 10 MB hard drive € 18.500,-
  
• Processor: Intel 8086-2 8 MHz (M24) / 8086-1 10 MHz (M24SP) with optional 8087 / Zilog Z8000 possible
  
• Memory: 128 KB to 640 KB DIP (max: 18xDIP16 256Kx1bit DRAM=512KB soldered, 18xDIP16 64Kx1bit DRAM=128KB socketed)
  
• Graphics Card: Olivetti Enhanced Color Graphics Adapter (eCGA), 32 KB memory
  
• Sound Card: PC-Speaker
  
• Display: Olivetti Monochrome Display, 10" green or amber, 640x400 monochrome / Olivetti Color Display, 12", 640x400, 16 colors
  
• Hard Drive: none to 20 MB MFM
  
• Floppy Drive: Standard 5.25" 360KB, up to 3.5" with 720 KB support (maximum of 2 drives)
  
• Ports: RGB, Parallel-Port, RS-232 25-pin DSUB, 9-pin DSUB Keyboard (proprietary)
  
• Keyboard: "Olivetti Keyboard 2" with 18 function keys and various special keys, otherwise compatible with IBM Enhanced Keyboard
  
• Mouse: Proprietary, RS232-incompatible 9-pin DSUB on the Olivetti Keyboard 2
  
• Battery Capacity: 60mAh for real-time clock
  
• Weight: 25kg
  

Software

BIOS
The BIOS version 1.43 or later can work with EGA graphics cards (this is the first version that allows the memory area to be cleared for third-party cards), generally with MS-DOS 5.0 and 720 KB 3.5" floppy drives. The BIOS is divided into two DIP chips (odd and even) and cannot be flashed. It should be noted that the jumper configuration in BIOS 1.43 differs from older versions regarding the memory configuration (more on this later).

The BIOS, even in its latest known revision 1.44, has the issue that the real-time clock can only store dates until 1992 (Olivetti allocated only 3 bits for the date). The driver CLOCK402.SYS, which needs to be loaded at startup, addresses this almost imperceptibly, so even in 2020, it's not an issue - at least with the original DOS 3.20. However, I must note that the combination of MS-DOS 5.0, Windows 2.x, and 3.x with the driver did NOT work; it caused freezes or "Divide by 0" error messages.

Additionally, the BIOS supports expansion cards with their own ROM, allowing for XT-IDE upgrades if the internal MFM hard drive is broken or not desired.

Operating System
The M24(SP) was last shipped with Olivetti-MS-DOS version 3.30. This version was developed in collaboration with Microsoft and is fully compatible with MS-DOS 3.30. It offers its own in-house software as replacements for fdisk.com, edlin.com, edit.com, etc., to make use of the Olivetti M24's hardware features. For example, Olivetti-MS-DOS 3.20 could already handle partitions over 32MB, whereas the retail MS-DOS 3.20 at that time could not.

Application Software
Along with Olivetti MS-DOS, the following software was included:

• KEYBOARD DRIVERS + UTILITIES M24 SP - Drivers for international keyboard layouts, tools like fdisk, and format.
  
• CUSTOMER TEST M24 (SP) - A diagnostic program that tests all hardware capabilities.
  
• GETTING TO KNOW THE M24 SP - A GWBASIC demo that shows how to operate the M24.

There was also a wide range of (paid) software distributed by Olivetti, such as a TSR-GUI called OLIMENU for DOS. It is similar to IBM TopView as a program launcher for DOS, but significantly simpler. Besides this, the entire software library available for an IBM-PC with an 808x CPU is at the user's disposal.

Games, Demos, and IBM PC Compatibility
Even though the M24 was regarded as "the" clone par excellence in terms of compatibility with the original IBM PC, with Microsoft's Flight Simulator  often being seen as the ultimate benchmark (or at least it was in the 80s), this is not entirely true, mainly due to the graphics card. For example, games like 7 Spirits of Ra  or demos like 8088 Domination  and 8088 Corruption  fail to run because they use low-level and hardware-specific and sometimes even undocumented tricks that only the original IBM CGA card can handle.

Hardware

CPU/FPU
The M24 came with an 8 MHz 8086-2, while the later M24SP featured a 10 MHz 8086-1.
Both could be equipped with a compatible 8087 coprocessor or NEC V20 or V30.

Memory
The motherboard can accommodate a total of 36 16-pin DIP chips (maximum capacity: 18xDIP16 256Kx1bit DRAM=512KB / 18xDIP16 64Kx1bit DRAM=128KB), providing a total of 640 KB of memory (depending on the revision, 128-512 KB on BANK0 is soldered, and 128-512 KB on BANK1 is socketed).

More than 640 KB of memory (such as through real mode or 8086-compatible CLEARMEM.SYS  in combination with USE!UMBS.SYS  for access to the Upper Memory Area, or UMA, between 640 KB and 1MB) is only possible with the use of expansion cards like the Monotech PCs MicroRAM and MS-DOS 5.0 or later. The 8086 can directly address a maximum of 1 MB. The MicroRAM can be activated in any UMA area without a ROM. In the case of my M24SP, this includes A000-B000, C000-D000, and E000-F000.

At the time, Olivetti offered a 16-bit memory expansion for the proprietary expansion sockets of the M24. A lucky owner on YouTube  showcases this ultra-rare Olivetti original card. As I wasn't wealthy enough for such original cards, thus I purchased an 8-bit MicroRAM card from Monotech , allowing me to have 630KB of free lower memory despite a ton of device drivers loaded upon boot. There were other third-party EMS memory expansions (according to LIM 4.0 standard) around 1990, typically working in the C000-D000 range with a 64 KB EMS page frame and bank switching, providing access to over 1 MB of memory. However, few cards back then allowed mapping the A000 to FFFF range as directly usable memory. This feature became more common with 386 machines, MS-DOS native support, and cheaper memory, notably with EMM386.EXE.

Graphics Card
The integrated graphics card can display a resolution of 640x400 pixels in two colors (monochrome), which exceeds the CGA standard (640x200 pixels), while still being generally compatible with pre-EGA graphics and text standards, except in cases of highly hardware-specific programming.

An additional expansion card, available for an extra cost, was the DEB "Display Enhancement Board": It could render a proprietary video mode at 640x400 with 16 colors, although it did not offer EGA compatibility. This card was so expensive and thus rare that it is now mostly known through the manual or the Windows 1.x setup. The corresponding monitor connects via a proprietary 25-pin DSUB connector, which, in the case of the monochrome monitor, also provides 25-volt power.

Hard Drive
The system has two bays for 5.25" half-height drives, one of which comes with a mounting frame for a 5.25" hard drive. Olivetti officially offered two hard drive models, with either 10 MB or 20 MB capacity. These used the MFM standard, common for devices of this era, where the controller recognized and supported only certain hard drive sizes.

Thanks to XT bus compatibility, it is also possible to retrofit IDE hard drive controllers with their own BIOS, such as XT-IDE, which I have done myself .

Floppy Drive
In its standard configuration, the system was shipped with a 5.25″ 360 KB floppy drive, but there was an option to install a second 5.25″ 360 KB floppy drive instead of a hard drive. It uses a flat ribbon cable, typical for desktop computers, so installing a 3.5" 1.44 MB floppy drive (in 720 KB mode) is not a problem, especially because it's supported by the BIOS.

Expansion Cards
The M24 featured 16-bit-wide interfaces for expansion cards, a rare but remarkable feature. The slots are compatible with the then-common 8-bit XT bus, but its full-size 16-bit cards have an 8-bit bus on both ends of the expansion cards, making later ISA 16-bit cards incompatible with the M24.

The integrated graphics card is one such example and has a unique characteristic: It also connects the upside-down hanging motherboard to the riser card through a 180-degree angle.

Additionally, there was a 16-bit Zilog Z8000 expansion card avilable. It makes the M24 compatible with the M20 (an UNIX machine).

Keyboard and Mouse
The keyboard connects to the M24 through a proprietary 9-pin DSUB (located right next to the power supply). It was marketed as the "Olivetti Keyboard 2". There were two versions, differing by the number of keys. The larger version, like mine, has 18 function keys.

The proprietary mouse is connected directly to the keyboard via another proprietary 9-pin DSUB connection. It's said that early Logitech mice were compatible with it. A normal RS232 mouse must not be used here - it causes a short circuit.

M24 Motherboard DIP Switches and Jumpers
The DIP switches and jumpers are described in the service manual (see downloads below). Note that the DIP switch configuration of BIOS 1.43 and newer differs from older BIOS versions regarding memory configuration. The linked service manual from that time addresses this (unlike an earlier Hadesnet link I had placed here).

Hardware Hacks and Tips

Using Non-Olivetti Monitors
If the original monitor is lost, aside from buying a replacement on eBay and other platforms, there are two options:

The RGBi Screen Variant
WARNING! The M24(SP) has a fixed line frequency of 25 KHz (640x400) in ALL resolution modes, which is simply too high for standard 15 KHz CGA screens of that time. Therefore, you cannot connect the M24 to monitors like the Philips 8833-II. The image will be distorted, and the Flyback Transformer (LOPT)  may be damaged. I had to repair my Philips monitor a week after testing out an adapter. Make sure to only use a "Multisync" screen that is capable of a 25 KHz input.

The DB25 connector on the graphics card has TTL output, hence a digital signal, also known as RGB-I. On the internet, particularly on the often-cited site M24 Hadesnet , a false PDF is circulating, which claims that PIN 7 is unassigned. However, this is where the essential INTENSITY bit for CGA is located. Additionally, PIN 2 must be connected to GND to activate the color mode on the graphics card. If this is not done, you will encounter the "underlined instead of blue" phenomenon described later.

The pinout for a multisync RGB-I monitor with a DB9 input is:

• M24 proprietary DB25 PIN# -> DB9 RGB PIN#
  
• 1 -> 8
  
• 3 -> 9
  
• 4 -> 3
  
• 5 -> 4
  
• 6 -> 5
  
• 7 -> 6
  
• 14 -> 1
  
• 15 -> 2
  

The VGA Monitor Variant
If the user can settle for 8 colors (the INTENSITY bit doesn't work on VGA, unless you get creative ), you can build the adapter described by M24 Hadesnet  with confidence. The reason this works is that the M24's 25 KHz line frequency is relatively close to VGA's 31 KHz, combined with how CGA operates, some VGA screens can handle this lower line frequency (one of my monitors can, the Samsung Syncmaster P2770HD).

Underlined Text Instead of Light Blue Color?
Owners of the monochrome monitor encounter a noticeable incompatibility with the IBM PC:

Apparently, even when using the VGA adapter mentioned above, there are lines in the image that shouldn't be there and some text might not be displayed as well. This occurs because the attribute for light blue of MDA (Monochrome Display Adapter) is interpreted as "underlined". This results in a dark blue underlined effect on the M24. According to Olivetti M24-240 FAQ - Retrocomputing.net , you just need to connect PIN 2 of the DB25 output to GND to switch the graphics card to the "true" color mode.

This actually allows light blue to be displayed. I tested it myself by connecting PIN 2 to the case. It works, and the spacing between the pins inside makes it easy to do and revert the change if needed.


Lost the original Olivetti M24 Mouse? Build your own!
If you're like me and don't want the proprietary mouse port to rust away or to keep the serial port free for TTY use, here's a tip: you can connect the commonly available Atari STM1 mouse to the Olivetti keyboard with a simple modification .

Date and Time Always Wrong?
The AT&T PC6300 / Olivetti M24(SP) has a real-time clock that can store the date with only 3 bits by design. As such, the M24 is one of the few computers not even being Year-1992-ready. In 1992, AT&T NCR released the CLOCK402.SYS driver to be loaded via CONFIG.SYS during boot. I found two versions online, however, both are incompatible with DOS 5.0 and later, leading to crashes when a program (like Windows CLOCK.EXE) tries to retrieve the BIOS time. As of 2024, I know of no replacement; XT-IDE and UMB users must set the time anew after every cold start, if they dont use mTCP's SNTP tool  during boot (AUTOEXEC.BAT), which retrieves date and time from a time server and sets it for DOS.

Freeze When Starting MS-DOS 5.0 and PC-DOS 5.0
If setup disks hang at startup, it's likely due to the KEYB.COM keyboard driver that comes with DOS 5 and higher. Removing it from AUTOEXEC.BAT on the setup disk allows the setup to start. As a replacement, you can use KEYBxx.EXE of DOS 3.x from the Keyboard Drivers and Utilities disk (which contains several language versions; it's part of the DOS 3.20 download linked below). KEYBGR.EXE is the German version. And it requires less memory on the other hand.

Power Supply Fan Extremely Loud
The 120x120mm fan (Model: ETRI 141 LW) in the M24 is powered by 230 volts (likely 110V in the AT&T PC6300 in the US) and consumes 10 watts on its own. It is entirely made of metal. Not only is this excessive for the required cooling, but it's also far too loud for my liking.
However, it is nearly impossible to find quiet fans on the market that are suitable for 230 volts (and let's not even discuss the unusual connections). These are typically fans for server farms or bathroom exhaust installations. I decided to replace the fan with a 23dB 12V silent fan and add a 3-pin potentiometer  for controlling the voltage and thereby the fan speed, which is connected via a splitter cable (Molex to Molex & 3-pin ) to the usual 12V Molex connector. As shown in the image to the left, the case has a passage to route cables from inside to outside. The rubber mounts on the existing fan must be cut because once they are pulled through a loop, there's no turning back. To replace the fan, it's necessary to remove the entire power supply. Be careful with high voltages inside.

Sporadic Crashes and Freezes in Various Programs
If the M24 starts experiencing sporadic crashes without a clear link to a specific program or process (like CLOCK402.SYS crashing whenever a program tries to access the date/time), it could be due to memory issues. If you also see the error DMA CONTROL FAIL:04 (as shown in the left image) when turning on the computer, it's an unmistakable sign. This error indicates a fault in the first 64KB of memory (source ).
The motherboard, as previously mentioned, has 18 soldered DIP16 chips of various sizes (128KB-512KB) and another 18 DIP16 chips that are socketed.
To troubleshoot, you can remove the socketed chips to see if the error persists. If the problem continues, then the next step might be piggybacking. This involves placing a defenitely functioning chip over a potentially faulty chip so that their pins make contact. If the soldered chip fails to provide data or provides incorrect data, the overlying chip might take over and provide the correct information. However, this doesn't always work, and the image linked here  from minuszerodegrees  demonstrates why. The technique usually works only if the faulty chip is completely dead with an interrupted circuit, simply causing the M24 to count less RAM on boot.

I piggybacked all 18 chips and gradually removed them to see if there were any more sporadic freezes or DMA Error 04. What can I say... when I removed the last chip, the M24 kept running stably. So, the faulty chip was randomly working throughout my test. In the end, I desoldered all the chips and installed sockets. A new set of chips finally ended the sporadic freezes.

Left image: Chips before desoldering, Middle: Chips after soldering sockets, Right: M24 working again

Screen Errors with MS-DOS 5.0/6.x and BIOS Before 1.43
BIOS versions before 1.43 often cause the following issues when running MS-DOS 5.0 and later:

• Screen output fails or is incorrect when using commands like DIR.
  
• The screen cannot be cleared with CLS.
  
• The MS-DOS Shell may not display or is displayed incorrectly.

The cause is an incompatibility with older BIOS versions and how graphics modes are handled starting from MS-DOS 5.0 (source ) BIOS 1.43 ROMs can still be found on eBay as of 2024. If you have an EPROM burner and two 8KB chips available, you can download BIOS 1.44 from here and burn them yourself.

DIR /P and MORE Keep Scrolling, and CLS Doesn't Clear the Screen (MS-DOS 5.0 and 6.x)
Despite having BIOS 1.43, you may still encounter this problem: With MS-DOS 5.0 and 6.x, you might notice that commands like dir /p and more don't pause after filling the screen; instead, they just keep scrolling. In some cases, CLS doesn't clear the screen either.

According to an old Microsoft Knowledge Base article, the solution is to include ANSI.SYS with the /S switch in CONFIG.SYS, or load PATCH.COM via AUTOEXEC.BAT. Since I use the original ANSI.SYS from Olivetti DOS 3.2, the /S switch doesn't work for me. Both solutions ultimately force MS-DOS into 25-line text mode.

The reason for this problem is how graphics modes are handled in DOS 5.0 and beyond. It uses the 40h:84h area of the BIOS to work in 43/30-line mode. Older MS-DOS versions didn't do this, so the Olivetti BIOS wasn't designed for it, returning an incorrect value to MS-DOS. The patch reconfigures this BIOS area to return the value 25 for CGA/MDA adapters. This way, DOS correctly recognizes a full screen and pauses after a full page (source ).

Downloads and Videos

Operating System and Applications

• Olivetti MS-DOS 3.20 along with drivers and application software  (WinImage/Teledisk images)
  
• BIOS 1.44 
  

Drivers

• ATTCLOCK.SYS January 14, 1992  (3 KB, ZIP) - Corrects the real-time clock (< DOS 3.3)
  
• ATTCLOCK.SYS January 9, 1992  (3 KB, ZIP) - Corrects the real-time clock (< DOS 5.0)
  
• A6_MOUSE Driver v.1.0 2/85  (91 KB, ZIP) - Olivetti Mouse MOUSE.COM driver for DOS, including demo programs (also for modified Atari STM1)
  
• M24 Controls  (7 KB, ZIP) - To change DOS graphics modes on the M24(SP)
  
• GOSLOW.COM / GOFAST.COM  (3 KB, ZIP) - Changes processor speed on M24SP between emulated 5MHz and its normal 10MHz.
  
• PATCH.COM  (1 KB, RAR) - Fixes the pagination issue from MS-DOS 5.0 onward.
  

Manual / User Guide

• AT&T PC 6300 Service Manual.pdf  (17.5 MB, RAR) - Contains jumper/switch configuration and troubleshooting