5.4CONFIGURATION REGISTER (CFG)

The CFG register is used to enable or disable various oper- ating modes and to control optional on-chip caches. Be- cause the CP3BT26 does not have cache memory, the cache control bits in the CFG register are reserved. All CFG bits are cleared on reset.

15

10

9

8

7

6

5

2

1

0

 

Reserved

SR

ED

0

0

Reserved

 

0

0

 

 

 

 

 

 

 

 

 

ED

The Extended Dispatch bit selects whether

 

the size of an entry in the interrupt dispatch ta-

 

ble (IDT) is 16 or 32 bits. Each entry holds the

 

address of the appropriate exception handler.

 

When the IDT has 16-bit entries, and all ex-

 

ception handlers must reside in the first 128K

 

of the address space. The location of the IDT

 

is held in the INTBASE register, which is not

 

affected by the state of the ED bit.

 

 

 

 

0 Interrupt dispatch table has 16-bit entries.

 

1 Interrupt dispatch table has 32-bit entries.

SR

The Short Register bit enables a compatibility

 

mode for the CR16B large model. In the

 

CR16C core, registers R12, R13, and RA are

extended to 32 bits. In the CR16B large mod- el, only the lower 16 bits of these registers are used, and these “short registers” are paired together for 32-bit operations. In this mode, the (RA, R13) register pair is used as the ex- tended RA register, and address displace- ments relative to a single register are supported with offsets of 0 and 14 bits in place of the index addressing with these displace- ments.

0 32-bit registers are used.

1 16-bit registers are used (CR16B mode).

CP3BT26

17

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National CP3BT26 Configuration Register CFG, When the IDT has 16-bit entries, and all ex, Mode for the CR16B large model