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SUPER EXTENDED BASIC UNRAVELLED II
COLOR COMPUTER 3
HARDWARE DIFFERENCES
ORIGIN: SPECTRAL ASSOC
REVISED:12/26/99 WALTER K ZYDHEK
COLOR COMPUTER 3 HARDWARE DIFFERENCES
This chapter deals with the major hardware and software differences between
the original Color Computer and the Color Computer 3. The designers of the Color
Computer 3 were guided by several design criteria, which occasionally forced some
odd decisions. First, the CoCo 3 had to be as compatible as was possible the older
CoCos so that as much of the old CoCo software as possible would function on the
CoCo 3. Also the CoCo 3 had to be as inexpensive as possible so that it would have
a market niche (other than just selling it to CoCo 2 owners). This constraint led
to the GIME chip (or custom or tequila chip as it was also known).
Memory
The most apparent difference with the Color Computer 3 is the capability of
having up to 512K of Random Access Memory (RAM). This RAM is made up of 256K
Dynamic RAM chips. The 128K version of the computer has four (64K x 4 bit) chips,
whereas the 512K version has sixteen (256K x 1 bit) chips. Upgrading from 128K to
512K primarily consists of removing the existing RAMs and inserting the 512K
upgrade board into the provided sockets.
It should be noted that several problems were encountered early on with the
RAMs supplied on early Color Computer 3s (the computer would crash during
manipulation of the screen in some of the horizontal virtual enable modes). These
models contained 150ns RAMs from Mitsubishi. These problems appeared to be solved
by replacing the RAMs with 150ns RAMs from Nippon Electronics Corp (NEC) or by
replacing them with 120ns parts.
The GIME Chip
The Color Computer 3 has many features not available on the original
Color Computer, including memory management, advanced interrupt processing, and
advanced graphics. All of these functions, in addition to the original Color
Computer graphics modes, are handled by one large chip referred to as the ‘GIME’
chip (pronounced “gimmee", for Graphics/Interrupt/Memory Enhancement). We will
touch on these subjects in the following paragraphs, and go into detail on them a
little later.
The Central Processing Unit (CPU) in the Color Computer 3 is the 6809.
This processor, by its very design, is limited to accessing 64K of memory at one
time. Making this chip work in a 512K computer is, therefore, a neat trick. To do
this, a system called memory management is employed. Memory within the computer is
divided up into 8K blocks (producing 16 blocks in a 128K system, 64 blocks in
512K). From this pool of 8K blocks you may select any 8 to fill the CPU’s memory
space of 64K.
Two additional interrupts have been added to the Color Computer 3. The
first is a timer interrupt which is a 12-bit interval timer, allowing you to set it
to any value from 0-4095. This timer is counted down, and when it goes below 0, an
interrupt may be triggered. The count is decremented every 70nsec or 63.5usec
(selectable). The other interrupt is a keyboard interrupt, causing an interrupt to
occur whenever a key or joystick button is pressed.
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