Jameco Electronics 3000, 2000 manual Memory Mapping

3.2 Memory Mapping

Although the Rabbit memory mapping scheme is fairly complex, the user rarely needs to worry about it because the details are handled by the Dynamic C development system.

Except for a handful of special instructions (see Section 19.5, “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 (IX, IY), program counter and stack pointer (SP)) are also 16 bits long.

Because Rabbit instructions use 16-bit addresses, the instructions are shorter and can exe- cute much faster than if, for example, 32-bit addresses were used. The executable code is very compact.

The Rabbit memory-mapping unit is similar to, but more powerful than, the Z180 mem- ory-mapping unit. Figure 3-2illustrates the relationship among the major components related to addressing memory.

Processor	Memory		Memory	Memory
	Mapping		Interface
				Chips
16		20
	Unit
				20 bits plus control
bits		bits

Figure 3-2. Addressing Memory Components

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 23