The Dimension 68000 is a microcomputer introduced by the Micro Craft Corporation in 1983 that emulates the Apple II, the IBM PC, and various CP/M-centric computers through a family of coprocessor expansion cards and emulation software. The Dimension 68000 can also run as a standalone computer based on the Motorola 68000 from which it gets its namesake. The computer is mostly the brainchild of Mike Carpenter, a former executive of a scientific instrument manufacturer who incorporated Micro Craft in Dallas, Texas, to develop the Dimension 68000. It had a market lifespan of three years and received mixed, mostly positive, reception from the technology press. Criticism was leveled at the $6,250 price tag for the computer with the full deck of coprocessor cards, as well as the extent of the emulation power of those cards.
Specifications Main unit and keyboard The Dimension 68000 is a desktop microcomputer whose main unit measures and weighs between , depending on the configuration. The main unit comprises a steel chassis with a cream-painted lid, a material chosen to resist RF interference. Early models had a chassis constructed from Noryl thermoplastic, painted cream on the outside and coated with conductive nickel paint on its underside. Six screws hold the lid on and can be unfastened for servicing. Micro Craft encouraged user-servicing from the onset and did not void the warranty of purchasers should they have opened the case, as was an increasing practice around the time of the Dimension's release. The dual 5.25-inch floppy disk drives and the accompanying full-drive-height blanking plate of the stock configuration feature black trim. Next to the disk drives is a reset button; the functionality of the button can be disabled in software to prevent accidental actuation. In the rear of the main unit are six expansion card holes which may be populated depending on the configuration (all six are populated with blanking plates in stock configuration), as well as one RS-232 serial port, a Centronics-style parallel port, the keyboard port, a joystick port, and the composite video output jack. A 100-watt, 15-amp switched-mode supply powers the unit and was described by Microcomputing magazine as "offer[ing] enough juice to power just about any board you'd want to add".
The motherboard of the Dimension 68000 comprises a four-layer printed circuit board. It rests on the bottom of the chassis and features six expansion slots. Among the other logic chips on the motherboard resides the main microprocessor—a Motorola 68000 clocked at 8 MHz—and the random-access, DIP memory chips. These chips takes up most of the footprint of the motherboard and are laid out as four banks of 17. The motherboard accepts a total of 512 KB of RAM on the motherboard. Additional RAM could be installed only on expansion cards; Micro Craft promised a memory expansion card in early 1984, for which multiple orders of the card could be used within the same machine and expand the computer's memory to 16 MB—the full amount of RAM that the Motorola 68000 can address. The stock configuration of the Dimension 68000 came only with 256 KB of RAM. An 8-KB read-only memory chip holds the bootstrap code, the Dimension 68000's BIOS, and basic diagnostic utilities. All the chips on the motherboard are socketed, rather than soldered.
The disk drives of the stock Dimension 68000 consist of two half-height, 5.25-inch, 40-track floppy drives. Two more half-height 5.25-inch floppy drives can be outfitted in the blank full-drive-height space to the right, or one full-height hard drive can be installed in the same space. The floppy drives are controlled by the NEC 765 chip, a versatile controller that supports numerous contemporary floppy disk encoding schemes, allowing it to read and format disks for the computer's IBM PC and CP/M-80 emulation modes. The Apple II emulation mode required its own unique floppy controller on the Apple II coprocessor card (see § Coprocessor cards). Hard drives also required their own controllers, included on expansion cards sold by Micro Craft with the drive or should the computer have been optioned with one; this controller card was not available in the first few months of the Dimension 68000's market availability. A utility included in the system disk also allows the floppy drives to read disks formatted in IBM PC CP/M, the TRS-80 range, all of Kaypro's computers, and single-density drives used by Cromemco and Osborne computers.
The computer has no prescribed monitor, and purchasers were expected to buy one separately. It did come packaged with a keyboard, built by Key Tronic and which features the same layout and interface as the keyboard for the IBM PC and PC XT. The display output of the stock machine generates composite video for both graphics and text. The text output was multi-mode and supported the following resolutions expressed as rows by columns of text: 20 by 20, 40 by 24, 80 by 24, and 50 by 25, non-interlaced; 80 by 50 and 100 by 50, interlaced. For graphics, the horizontal resolution of the stock machine is limited to 525-lines of NTSC video. An RGB expansion card for digital component video output was introduced sometime between July 1984 and April 1985.
Expansion bus, coprocessor cards, and emulation modes The Dimension 68000's coprocessor expansion cards were the main draw of the computer because it promised a cross-platform experience in one unit. Regardless, purchasers could have ordered a Dimension 68000 without any cards and have it still be functional as a 68000-based computer, albeit with a limited selection of turnkey applications. The Dimension 68000's expansion bus is of Micro Craft's own proprietary design, although its specification was open to prospective third-party manufacturers, as well as a development-kit version of the computer with a prototyping perfboard included. The bus connectors are 86 pins wide, allowing access to the computer's 16-bit data path, 24-bit address lines, and the Motorola 68000's major and memory-access signals, as well as providing individual direct memory access and interrupt capabilities for each slot. The bus also conveys all disk drive signals, allowing expansion cards to equip disk controllers besides the built-in NEC 765; such is the case with the Apple II coprocessor card.
The Dimension 68000 was described as a "hybrid emulator"—that is, each of the coprocessors performed the bulk of code execution in their respective emulation modes, while certain memory-resident programs handled elements of emulation in the background. The Motorola 68000 additionally handled certain interrupts the coprocessors by themselves could not execute—chiefly regarding display output and disk reading and writing.
From its inception through to its demise, Micro Craft only offered three expansion cards for the Dimension 68000—an IBM PC emulation card, an Apple II emulation card, and a CP/M card for Zilog Z80–based machines such as the Kaypro and Osborne 1. In order to use any of these emulation cards, an "emulation master" disk comprising the specific files included in the master system disk had to be written to a user's spare floppy disk, in a process outlined in the copy utility included in the master disk and in the Dimension 68000's manual. This "emulation master" disk is necessary to boot from any of the three emulation cards. The user then has to insert the system disk into drive A and the "emulation master" disk into drive B and type into the default system prompt, e.g., , to boot into the IBM PC emulation mode. Both disks could then be removed from the drives, and specific IBM PC software can be loaded from disk.
Apple II card The Apple II coprocessor card runs off an MOS 6512, as opposed to the real Apple II's MOS 6502, because of the former's reliance on an external clock generator—a setup better suited for multiprocessor arrangements such as the Dimension 68000. A bespoke floppy controller is also included on the Apple II coprocessor card, necessary for allowing the floppy drive motor to move in half-steps among tracks, a feature favored by certain Apple II software developers to copy-protect their commercial releases. Copy protection schemes relying on specific timings of the Apple II's disk loader routine, on the other hand, may cause some software to fail to load, because of the Motorola 68000's handling of certain interrupts during this routine.
As opposed to loading IBM emulation mode, which only requires the user enter at the Dimension 68000's native CP/M prompt, the Dimension 68000's Apple II emulation mode requires the user enter several arguments after . These arguments specify the presence of a printer, how many disk drives if any are to be emulated, the type of keyboard, the type of display, and so on.
The Apple II emulation mode additionally requires the user provide their own copy of an operating system, such as Apple DOS or ProDOS. Once booted into a Applesoft BASIC prompt, however, the interpreter recognizes all BASIC syntax, down to the s and s. As well, the Apple II coprocessor card emulates a full deck of Apple II peripheral cards: in slot 0 is the 16 KB Language Card; in slot 1 is the parallel card; in slot 2 is the serial card; in slot 3 is the 80-Column Text Card; in slot 6 is the floppy controller, with two virtual Apple disk drives plugged in; and in slot 7 is the hard drive controller card. Applesoft BASIC can be used to invoke any of these cards with expected behavior; for example, to load the 80-Column Text Card, the user enters at the prompt.
IBM PC card The IBM PC coprocessor card runs off an Intel 8086 clocked at 7.3 MHz, slightly less than twice the speed of the IBM PC's 4.77-MHz Intel 8088 (a variant of the 8086 with an eight-bit external data bus). This faster clock speed was chosen to compensate for any slowdown caused by software emulation and interrupt handling by the Motorola 68000. The resulting speed of calculations performed within spreadsheet software was comparable to a real IBM PC's speed, according to Popular Computing.
The standard emulation program on the IBM "emulation master" disk emulates an IBM PC with a CGA monitor displaying text at 80 columns by 25 rows. Three more emulation programs are located on disk. The first, , emulates a PC with a CGA monitor displaying text at 40 columns by 25 rows. The second, , emulates a PC with a Monochrome Display Adapter monitor displaying text at 80 columns by 25 rows but incapable of displaying any graphics. The last, , was an experimental "enhanced" IBM emulation mode, version 1.18, which as of September 1984 had not been fully debugged; it added direct I/O to the NEC 765 and expanded the capability of the RS-232 serial port under IBM PC emulation. All four of the aforementioned emulation programs can be run without passing any arguments, unlike with . In this way, the emulation programs defaulted to running the maximum amount of RAM possible, minus some reserved for the emulation software. Running (CGA, 80-by-25 text) on a machine equipped with 512 KB of RAM, for example, left 412 KB addressable to the virtual machine. Some arguments could be passed to these emulation programs: limited the memory to a set amount; rendered characters with the intensity bit set to high as reverse video (see IBM Monochrome Display Adapter § Capabilities); and is a software analogue to the DIP switches on an IBM PC's motherboard, used to set certain hardware parameters.