the root segment or it may contain data variables. The stack segment is normally 4K long and it holds the system stack. The XPC segment is normally used to execute code that is not stored in the root segment or the data segment. Special instructions support executing code that is visible in the XPC segment.

The memory interface unit receives the 20-bit addresses generated by the memory-map- ping unit. The memory interface unit conditionally modifies address lines A16, A18 and A19. The other address lines of the 20-bit address are passed unconditionally. The mem- ory interface unit provides control signals for external memory chips. These interface sig- nals are chip selects (/CS0, /CS1, /CS2), output enables (/OE0, /OE1), and write enables (/WE0, /WE1). These signals correspond to the normal control lines found on static mem- ory chips (chip select or /CS, output enable or /OE, and write enable or /WE). In order to generate these memory control signals, the 20-bit address space is divided into four quad- rants of 256K each. A bank control register for each quadrant determines which of the chip selects and which pair of output enables, and write enables (if any) is enabled when a memory read or write to that quadrant takes place. For example, if a 512K x 8 flash mem- ory is to be accessed in the first 512K of the 20-bit address space, then /CS0, /WE0, /OE0 could be enabled in both quadrants.

Figure 3-4shows a memory interface unit.

Axxin—from processor

 

 

 

 

 

 

 

Axx—out from memory

A19in

 

 

control unit

 

 

 

 

 

 

 

 

 

Address lines not shown

 

A18in

 

 

 

are passed directly.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

A19in A19in'

A18in

A19

A18, A19 invertible

A18 by quadrant

/CS0

memory /CS1 control

/CS2

lines

Optional A19 inversion

memory

 

control

Read/Write

Synchronization

/OE0

/WE0

/OE1

/WE1

Figure 3-4. Memory Interface Unit

User’s Manual

25

Page 34
Image 34
Jameco Electronics 3000, 2000 manual 4shows a memory interface unit