Memory Mapping - Rabbit 2000 User Manual

3.2 Memory Mapping

Except for a handful of special instructions (see Section 18.5, "16-bit Load and Store 20-
bit Address"), the Rabbit instructions directly address a 64K data memory space. This
means that the address fields in the instructions are 16 bits long and that the registers that
may be used as pointers to memory addresses (index registers (
and stack pointer (
SP
Because Rabbit instructions use 16-bit addresses, the instructions are shorter and can exe-
cute much faster than, for example, 32-bit addresses. The executable code is also very
compact. Even though these 16-bit addresses are a valuable asset, they do create some
complications because a memory-mapping unit is needed in order to access a reasonable
amount of memory for modern C programs.
The Rabbit memory-mapping unit is similar to, but more powerful than, the Z180 mem-
ory-mapping unit. Figure 3-2 illustrates the relationship among the major components
related to addressing memory.
Processor
The memory-mapping unit receives 16-bit addresses as input and outputs 20-bit addresses.
The processor (except for certain LDP instructions) sees only a 16-bit address space. That
is, it sees 65536 distinctly addressable bytes that its instructions can manipulate. Three
segment registers are used to map this 16-bit space into a 1-megabyte space. The 16-bit
space is divided into four separate zones. Each zone, except the first or root zone, has a
segment register that is added to the 16-bit address within the zone to create a 20-bit
address. The segment register has eight bits and those eight bits are added to the upper
four bits of the 16-bit address, creating a 20-bit address. Thus, each separate zone in the
16-bit memory becomes a window to a segment of memory in the 20-bit address space.
The relative size of the four segments in the 16-bit space is controlled by the SEGSIZE
register. This is an 8-bit register that contains two 4-bit registers. This controls the bound-
ary between the first and the second segment and the boundary between the second and
the third segment. The location of the two movable segment boundaries is determined by a
4-bit value that specifies the upper four bits of the address where the boundary is located.
These relationships are illustrated in Figure 3-3.
User's Manual
)) are also 16 bits long.
Memory
Mapping
16
Unit
bits
Figure 3-2. Addressing Memory Components
IX
Memory
Interface
20
bits
, ), program counter
IY
Memory
Chips
20 bits plus control
15