User’s Manual
7
2. R
ABBIT
D
ESIGN
F
EATURES
The Rabbit is an evolutionary design. The instruction set and the register layout is that of
the Z80 and Z180. The instruction set has been augmented by a substantial number of new
instructions. Some obsolete or redundant Z180 instructions have been dropped to make
available efficient 1-byte opcodes for important new instructions. (see “Differences Rabbit
vs. Z80/Z180 Instructions” on page 187.) The advantage of this evolutionary approach is
that users familiar with the Z80 or Z180 can immediately understand the Rabbit. Existing
source code can be assembled or compiled for the Rabbit with minimal changes.
Changing technology has made some features of the Z80/Z180 family obsolete, and these
have been dropped. For example, the Rabbit has no special support for dynamic RAM but
it has extensive support for static memory. This is because the price of static memory has
decreased to the point that it has become the preferred choice for medium-scale embedded
systems. The Rabbit has no support for DMA (direct memory access) because most of the
uses for which DMA is traditionally used do not apply to embedded systems, or they can
be accomplished better in other ways, such as fast interrupt routines, external state
machines or slave processors.
Our experience in writing C compilers has revealed the shortcomings of the Z80 instruc-
tion set for executing the C language. The main problem is the lack of instructions for han-
dling 16-bit words and for accessing data at a computed address, especially when the stack
contains that data. New instructions correct these problems.
Another problem with many 8-bit processors is their slow execution and a lack of number-
crunching ability. Good floating-point arithmetic is an important productivity feature in
smaller systems. It is easy to solve many programming problems if an adequate floating-
point capability is available. The Rabbit’s improved instruction set provides fast floating-
point and fast integer math capabilities.
The Rabbit supports four levels of interrupt priorities. This is an important feature that
allows the effective use of super fast interrupt routines for real-time tasks.
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