Commodore REU
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Commodore Commodore may refer to: Ranks * Commodore (rank), a naval rank ** Commodore (Royal Navy), in the United Kingdom ** Commodore (United States) ** Commodore (Canada) ** Commodore (Finland) ** Commodore (Germany) or ''Kommodore'' * Air commodore ...
's RAM Expansion Unit (REU) range of external
RAM Ram, ram, or RAM may refer to: Animals * A male sheep * Ram cichlid, a freshwater tropical fish People * Ram (given name) * Ram (surname) * Ram (director) (Ramsubramaniam), an Indian Tamil film director * RAM (musician) (born 1974), Dutch * ...
add-ons for their
Commodore 64 The Commodore 64, also known as the C64, is an 8-bit home computer introduced in January 1982 by Commodore International (first shown at the Consumer Electronics Show, January 7–10, 1982, in Las Vegas). It has been listed in the Guinness ...
/ 128 home computers was announced at the same time as the C128. The REUs came in three models, initially the 1700 (128  KB) and 1750 (), and later the 1764 (, for the C64). 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
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(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 ) 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 KERNAL is Commodore's name for the ROM-resident operating system core in its 8-bit home computers; from the original PET of 1977, followed by the extended but strongly related versions used in its successors: the VIC-20, Commodore 64, Plus/4, ...
and
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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

Although the C128 could access more than of RAM through
bank switching Bank switching is a technique used in computer design to increase the amount of usable memory beyond the amount directly addressable by the processor instructions. It can be used to configure a system differently at different times; for example ...
, 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 Commodore BASIC, also known as PET BASIC or CBM-BASIC, is the dialect of the BASIC programming language used in Commodore International's 8-bit home computer line, stretching from the PET of 1977 to the C128 of 1985. The core is based on 6502 ...
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 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 Dynamic random-access memory (dynamic RAM or DRAM) is a type of random-access semiconductor memory that stores each bit of data in a memory cell, usually consisting of a tiny capacitor and a transistor, both typically based on metal-oxid ...
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 . 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. During the late 1980s, there was a shortage of DRAM caused by the
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enacting anti- dumping restrictions on Japanese manufacturers, thus 1750s were rare and expensive. However it was comparatively easy to upgrade a 1700 or 1764 to . Several firms did this commercially, either selling upgraded units or upgrading customer-supplied units. In the early 1990s,
DIY "Do it yourself" ("DIY") is the method of building, modifying, or repairing things by oneself without the direct aid of professionals or certified experts. Academic research has described DIY as behaviors where "individuals use raw and sem ...
modification schemes to increase the capacity of an REU to one
megabyte The megabyte is a multiple of the unit byte for digital information. Its recommended unit symbol is MB. The unit prefix ''mega'' is a multiplier of (106) in the International System of Units (SI). Therefore, one megabyte is one million bytes o ...
or higher appeared on various online services.


Model differences

The 1700s
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was identical to that of the 1750, and a
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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 A resistor is a passive two-terminal electrical component that implements electrical resistance as a circuit element. In electronic circuits, resistors are used to reduce current flow, adjust signal levels, to divide voltages, bias active el ...
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 In computing, a memory address is a reference to a specific memory location used at various levels by software and hardware. Memory addresses are fixed-length sequences of digits conventionally displayed and manipulated as unsigned integers. Su ...
$DF00's
bit The bit is the most basic unit of information in computing and digital communications. The name is a portmanteau of binary digit. The bit represents a logical state with one of two possible values. These values are most commonly represente ...
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 Commodore may refer to: Ranks * Commodore (rank), a naval rank ** Commodore (Royal Navy), in the United Kingdom ** Commodore (United States) ** Commodore (Canada) ** Commodore (Finland) ** Commodore (Germany) or ''Kommodore'' * Air 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
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community. Programs such as Color64 required a REU to minimize load times when switching between sections of the BBS. The REUs came with software to use the extra memory as a
RAM disk Ram, ram, or RAM may refer to: Animals * A male sheep * Ram cichlid, a freshwater tropical fish People * Ram (given name) * Ram (surname) * Ram (director) (Ramsubramaniam), an Indian Tamil film director * RAM (musician) (born 1974), Dutch * Ra ...
, but the RAM disk's compatibility with commercial software varied as some commercial software relied heavily on various quirks of the
Commodore 1541 The Commodore 1541 (also known as the CBM 1541 and VIC-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 ...
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 Ram, ram, or RAM may refer to: Animals * A male sheep * Ram cichlid, a freshwater tropical fish People * Ram (given name) * Ram (surname) * Ram (director) (Ramsubramaniam), an Indian Tamil film director * RAM (musician) (born 1974), Dutch * Ra ...
, as did the C128's version of
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, 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 Computer data storage is a technology consisting of computer components and recording media that are used to retain digital data. It is a core function and fundamental component of computers. The central processing unit (CPU) of a computer ...
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 In computer architecture, a bus (shortened form of the Latin '' omnibus'', and historically also called data highway or databus) is a communication system that transfers data between components inside a computer, or between computers. This ex ...
—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 fetch The instruction cycle (also known as the fetch–decode–execute cycle, or simply the fetch-execute cycle) is the cycle that the central processing unit (CPU) follows from boot-up until the computer has shut down in order to process instruction ...
es and only one quarter or less on payload data.


See also

* Super 1750 Clone *
geoRAM geoRAM from Berkeley Softworks was a memory expansion peripheral for use on the Commodore 64 computer with GEOS operating system. geoRAM was created by Dave Durran. During the chip shortages of the 1980s, Commodore could not produce enough of its ...
*Creative Micro Designs RAMLink *
Creative Micro Designs Creative Micro Designs (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 ...
2 MB REU, (1750 XL)


References


External links


REU Games & UtilitiesREU Programming documentationSource code of RAMDOS, a RAM disk program for the Commodore RAM Expansion UnitREU Programming - by Robin Harbron
{{DEFAULTSORT:Commodore Reu Memory expansion
REU Reu or Ragau ( he, רְעוּ, Rə'ū; grc-x-biblical, Ῥαγαύ, Rhagaú), according to Genesis in the Hebrew Bible, was the son of Peleg and the father of Serug, thus being Abraham's great-great-grandfather and the ancestor of the Israelite ...
Home computer peripherals Commodore 64