Jameco Electronics 3000 ユーザーズマニュアル

118

Rabbit 3000 Microprocessor

Figure 8-4. Memory Segments

The memory management unit accepts a 16-bit address from the processor and translates
it into a 20-bit address. The procedure to do this works as follows.

1. It is determined which segment the 16-bit address belongs to by inspecting the upper 4

bits of the address. Every address must belong to one of the possible 4 segments.

2. Each segment has an 8-bit segment register. The 8-bit segment register is added to the

upper 4 bits of the 16-bit address to create a 20-bit address. Wraparound occurs if the
addition would result in an address that does not fit in 20 bits.

Table 8-1. Segment Registers

Segment Register

Function

XPC

Locates extended code segment in physical memory. Read and written by
processor instructions: ld a,xpc, ld xpc,a, lcall, lret, ljp

STACKSEG = 0x11

Locates stack segment in physical memory.

DATASEG = 0x12

Locates data segment in physical memory.

Table 8-2. Segment Size Register

Bits 7..4

Bits 3..0

SEGSIZE = 0x13

Boundary address stack segment.

Boundary address data segment.

Extended code
XPC segment
(8K)

Stack segment
(4K typ)

Root segment

Data segment

64K

Boundary SEGSIZE[4..7]

Boundary SEGSIZE[0..3]

XPC

STACKSEG

DATASEG

16-bit address

20-bit address