Commodore REU

Commodore REU
Manufacturer	Commodore Business Machines (CBM)
Type	Memory expansion
Release date	1986;38 years ago
Discontinued	1990;34 years ago
CPU	MOS 8726 REC (RAM Expansion Controller) DMA ASIC
Memory	128 KB RAM (1700); 256 KB RAM (1764); 512 KB RAM (1750);
Storage	4164 64kx1 or 41256 256kx1 DRAM ICs

Last updated March 02, 2024

The Commodore RAM Expansion Unit (REU) is a range of external RAM add-ons.

The need for the REU came about when Commodore management decided to not use the final version of the custom Memory Management Unit (MMU) which then limited the size of memory in spite of early discussion of a larger memory map. Engineers traveling to the 1985 Consumer Electronics Show (CES) were confronted with flyers and billboards advertising a memory size that was no longer supported and finally the top management asked where the additional memory (up to 512 KB) would plug in.

By the time of the 1985 CES show in Chicago, the engineers were able to display a spinning globe of the earth as a demonstration of Direct Memory Access (DMA) by the new REU units.

The REU hardware was designed by Frank Palia and the dedicated RAM Expansion Controller (REC) integrated circuit (IC) was designed by Victor Andrade. Fred Bowen and Terry Ryan adapted the C128's KERNAL and BASIC to accommodate the REU natively and Hedley Davis wrote the globe spinning demo which was an impressive display of animation in the mid-1980s.

Hardware description

Official performance demonstration Globe.gif — Official performance demonstration

Although the C128 could access more than 64 KB of RAM through bank switching, the memory inside the REU could only be accessed by memory-transfers (STORE/LOAD/SWAP/COMPAREs) between the main memory and the REU memory. Additionally, the C128's built-in BASIC 7.0 had three statements, STASH, FETCH, and SWAP, for storing and retrieving data from the REU.

Officially, only the 1700 and 1750 were supported on the C128. The 256 KB model, the 1764, was released for the C64 at the same time. There were only minor differences between the three models. The original C64 power supply could not support the additional load presented by the 1764 so a 2.5 ampere C64 power supply was included.

The 1700 uses sixteen 4164 64kx1 DRAM ICs, the 1764 has eight 41256 256kx1 DRAMs, and the 1750 has sixteen 41256 DRAMs.

In practice, the difference between the 1764 and the earlier units had little effect on compatibility, and people used 1700s and 1750s successfully with the C64, and 1764s successfully with the C128, although the C64's stock power supply was inadequate to reliably handle the power load of any of them. Some dealers unbundled the 1764 and the power supply in order to sell the power supply to C64 users, and/or upgrade the 1764 to 512 KB.

The REU model 1750 can be modified to support up to 2MB on memory by installing 2 additional ICs for bank switch. There is only one source of software incompatibility in this modification: software that doesn't set the bits at $DF06 to the same value at all times, or sets the bits to a bank of 512 kb that isn't installed yet, won't work.^[2]

During the late 1980s, there was a shortage of DRAM thus 1750s were rare and expensive. However it was comparatively easy to upgrade a 1700 or 1764 to 512 KB. Several firms did this commercially, either selling upgraded units or upgrading customer-supplied units.

In the early 1990s, DIY modification schemes to increase the capacity of an REU to one megabyte or higher appeared on various online services.

Model differences

The 1700s circuit board was identical to that of the 1750, and a trace marked J1 indicated the size of the chips used. On the 1750 and 1764, this trace was cut.

The 1700 and 1750 had a resistor at position R4 that, according to Commodore engineer Fred Bowen, compensated for subtle timing differences in the expansion port on the C64 and C128. The 1764 lacked that resistor. Bowen and other CBM engineers recommended against using a 1764 with a C128 unless the resistor was added, or a 1700/1750 with a C64 unless the resistor was removed.

It was possible to check for the presence of a 1750 by reading memory address $DF00's bit 4, which was 1 on a 1750, and 0 on a 1700 or 1764. However, since this procedure would not distinguish between a 1700 and a 1764. To solve this, programmers wrote to, then read from, the REU's RAM to find out the amount of memory installed.

REU software support

A small percentage of software made use of the REUs. Like other add-on products from Commodore, their relatively small installed base relative to the huge number of C64s & C128s made software developers hesitant to invest much time and effort in supporting REUs. The lack of commercial support kept sales lower than they otherwise might have been. Due to its high speed relative to Commodore's floppy drives or even the commercially available hard drives, REUs did see support in the Commodore BBS community. Programs such as Color64 required a REU to minimize load times when switching between sections of the BBS.^[3]

The REUs came with software to use the extra memory as a RAM disk, but the RAM disk's compatibility with commercial software varied as some commercial software relied heavily on various quirks of the Commodore 1541 floppy drive. Additionally, many commercial programs simply overwrote the memory space occupied by the RAM disk software.

The GEOS operating system had built in support for the REU as a RAM disk, as did the C128's version of CP/M, and some disk copy programs used the REU to facilitate high-speed copying with a single disk drive. GEOS as well as other programs even used the REU for quick memory transfers within the host machine's main memory by storing a memory block into the REU and then fetching it back to another location. Using this method, only the actual data to be transferred needed to travel on the machine's data bus —unlike the ordinary method, which had the computer's CPU do the transfer, thus spending at least three quarters of the bus capacity on instruction fetches and only one quarter or less on payload data.

Related Research Articles

Atari ST is a line of personal computers from Atari Corporation and the successor to the Atari 8-bit family. The initial model, the Atari 520ST, had limited release in April–June 1985 and was widely available in July. It was the first personal computer with a bitmapped color GUI, using a version of Digital Research's GEM from February 1985. The Atari 1040ST, released in 1986 with 1 MB of RAM, was the first home computer with a cost-per-kilobyte of less than US$1.

<span class="mw-page-title-main">Commodore 1541</span> External 5 1/4-inch floppy disk drive for the Commodore C64 homecomputer

The Commodore 1541 is a floppy disk drive which was made by Commodore International for the Commodore 64 (C64), Commodore's most popular home computer. The best-known floppy disk drive for the C64, the 1541 is a single-sided 170-kilobyte drive for 5¼" disks. The 1541 directly followed the Commodore 1540.

<span class="mw-page-title-main">Commodore 64</span> 8-bit home computer introduced in 1982

The Commodore 64, also known as the C64, is an 8-bit home computer introduced in January 1982 by Commodore International. It has been listed in the Guinness World Records as the highest-selling single computer model of all time, with independent estimates placing the number sold between 12.5 and 17 million units. Volume production started in early 1982, marketing in August for US$595. Preceded by the VIC-20 and Commodore PET, the C64 took its name from its 64 kilobytes(65,536 bytes) of RAM. With support for multicolor sprites and a custom chip for waveform generation, the C64 could create superior visuals and audio compared to systems without such custom hardware.

<span class="mw-page-title-main">Commodore 1571</span> Floppy disk drive

The Commodore 1571 is Commodore's high-end 5¼" floppy disk drive, announced in the summer of 1985. With its double-sided drive mechanism, it has the ability to use double-sided, double-density (DS/DD) floppy disks, storing a total of 360 kB per floppy. It also implemented a "burst mode" that improved transfer speeds, helping address the very slow performance of previous Commodore drives.

<span class="mw-page-title-main">Commodore 128</span> Home computer released in 1985

The Commodore 128, also known as the C128, C-128, or C= 128, is the last 8-bit home computer that was commercially released by Commodore Business Machines (CBM). Introduced in January 1985 at the CES in Las Vegas, it appeared three years after its predecessor, the Commodore 64, the bestselling computer of the 1980s. Approximately 2.5 million C128s were sold during its four year production run.

<span class="mw-page-title-main">VIC-20</span> 1981 home computer by Commodore

The VIC-20 is an 8-bit home computer that was sold by Commodore Business Machines. The VIC-20 was announced in 1980, roughly three years after Commodore's first personal computer, the PET. The VIC-20 was the first computer of any description to sell one million units. It was described as "one of the first anti-spectatorial, non-esoteric computers by design...no longer relegated to hobbyist/enthusiasts or those with money, the computer Commodore developed was the computer of the future."

The Commodore PET is a line of personal computers produced starting in 1977 by Commodore International. A single all-in-one case combines a MOS Technology 6502 microprocessor, Commodore BASIC in read-only memory, keyboard, monochrome monitor, and, in early models, a cassette deck.

The Commodore 16 is a home computer made by Commodore International with a 6502-compatible 7501 or 8501 CPU, released in 1984 and intended to be an entry-level computer to replace the VIC-20. A cost-reduced version, the Commodore 116, was mostly sold in Europe.

The Commodore Plus/4 is a home computer released by Commodore International in 1984. The "Plus/4" name refers to the four-application ROM-resident office suite ; it was billed as "the productivity computer with software built in".

Creative Micro Designs, Inc. (CMD) was founded in 1987 by Doug Cotton and Mark Fellows. It is a computer technology company which originally developed and sold products for the Commodore 64 and C128 8-bit personal computers. After 2001 it sold PCs and related equipment.

<span class="mw-page-title-main">Macintosh 512K</span> Revised version of the Macintosh 128K by Apple Computer

The Macintosh 512K is a personal computer that was designed, manufactured and sold by Apple Computer from September 1984 to April 1986. It is the first update to the original Macintosh 128K. It was virtually identical to the previous Macintosh, differing primarily in the amount of built-in random-access memory. The increased memory turned the Macintosh into a more business-capable computer and gained the ability to run more software. It is the earliest Macintosh model that can be used as an AppleShare server and, with a bridge Mac, communicate with modern devices.

<span class="mw-page-title-main">Commodore 65</span> Prototype computer

The Commodore 65 is a prototype computer created at Commodore Business Machines in 1990–1991. It is an improved version of the Commodore 64, and it was meant to be backwards-compatible with the older computer, while still providing a number of advanced features close to those of the Amiga.

The Commodore SX-64, also known as the Executive 64, or VIP-64 in Europe, is a portable, briefcase/suitcase-size "luggable" version of the popular Commodore 64 home computer and the first full-color portable computer.

The Commodore CBM-II series is a short-lived range of 8-bit personal computers from Commodore Business Machines (CBM), released in 1982 and intended as a follow-on to the Commodore PET series.

BASIC 8 – "The Enhanced Graphics System For The C128" – was an American-designed graphics system developed by Walrusoft of Gainesville, Florida and published in 1986 by Patech Software of Somerset, New Jersey. The system was an extension of Commodore's BASIC 7.0 for the C128 home/personal computer. BASIC 8.0 provided commands lacking in BASIC 7.0 to generate (color) graphics in the C128's high-resolution 80-column mode for RGB monitors.

<span class="mw-page-title-main">GEOS (8-bit operating system)</span>

GEOS is a discontinued operating system from Berkeley Softworks. Originally designed for the Commodore 64 with its version being released in 1986, enhanced versions of GEOS later became available in 1987 for the Commodore 128 and in 1988 for the Apple II series of computers. A lesser-known version was also released for the Commodore Plus/4.

The Super 1750 Clone was a 512 kB RAM expansion unit designed as a tiny, but compatible, third-party replacement for Commodore's then out-of-production CBM 1750 REU. Manufactured by Chip Level Designs, the Super 1750 Clone was sold by Software Support International.

<span class="mw-page-title-main">Commodore 64 peripherals</span>

The Commodore 64 home computer used various external peripherals. Due to the backwards compatibility of the Commodore 128, most peripherals would also work on that system. There is also some compatibility with the VIC-20 and Commodore PET.

geoPublish is a discontinued desktop publishing program made by Berkeley Softworks for their GEOS Operating System.

RAM pack, RAMpack, RAM expansion cartridge, RAM expansion unit (REU), memory expansion pak and memory module are some of the most common names given to various self-contained units or cartridges that expand a computer, games console or other device's own internal RAM in a user-friendly manner.

References

↑ Parallelus. "Commodore 64 - The Best Selling Computer In History - Commodore Computers: VIC20 C64 PET C128 Plus4 - All 8 Bit Machines". Commodore Computers: VIC20 C64 PET C128 Plus4 - All 8 Bit Machines. Retrieved 2016-02-08.
↑ DrWatson (2022-11-24). "Beyond 512 kb: The Two Megabyte REU". Neperos. Retrieved 2022-11-25.
↑ Zimmerman. "Color 64/v128 Preview Page". Bo Zimmerman. Retrieved 2018-11-12.

External links

This page is based on this Wikipedia article
Text is available under the CC BY-SA 4.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.

[1] Parallelus. "Commodore 64 - The Best Selling Computer In History - Commodore Computers: VIC20 C64 PET C128 Plus4 - All 8 Bit Machines". Commodore Computers: VIC20 C64 PET C128 Plus4 - All 8 Bit Machines. Retrieved 2016-02-08.

[2] DrWatson (2022-11-24). "Beyond 512 kb: The Two Megabyte REU". Neperos. Retrieved 2022-11-25.

[3] Zimmerman. "Color 64/v128 Preview Page". Bo Zimmerman. Retrieved 2018-11-12.

[1]

[2]

[3]