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TI486 Microprocessor - Al Kossow's Bitsavers

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Register Set<br />

Table 2-5. eRO Bit Definitions<br />

BIT<br />

POSITION<br />

NAME<br />

FUNCTION<br />

0 PE Protected Mode Enable. Enables the segment based protection mechanism. If PE=1,<br />

protected mode is enabled. If PE=O, the CPU operates in real mode, with segment based<br />

protection disabled, and addresses are formed as in an 8086-class CPU.<br />

1 MP Monitor Processor Extension. If MP=1 and TS=1, a WAIT instruction causes fault 7. The TS<br />

bit is set to 1 on task switches by the CPU. Floating-point instructions are not affected by the<br />

state of the MP bit. The MP bit should be set to one during normal operations.<br />

2 EM Emulate Processor Extension. If EM=1, all floating-point instructions cause a fault 7.<br />

3 TS Task Switched. Set whenever a task switch operation is performed. Execution of a<br />

floating-point instruction with TS=1 causes a device not available (DNA) fault. If MP=1 and<br />

TS=1, a WAIT instruction also causes a DNA fault.<br />

4 1 Reserved. Do not attempt to modify.<br />

5 0 Reserved. Do not attempt to modify.<br />

16 WP Write Protect. Protects read-only pages from supervisor write access. The 386-type CPU<br />

allows a read-only page to be written from privilege level 0-2. The <strong>TI486</strong> CPU is compatible<br />

with the 386-type CPU when WP=O. WP=1 forces a fault on a write to a read-only page from<br />

any privilege level.<br />

18 AM <strong>Al</strong>ignment Check Mask. If AMc:1 , the AC bit in the EFLAGS register is unmasked and allowed<br />

to enable alignment check faults. Setting AM=O prevents AC faults from occurring.<br />

29 0 Reserved. Do not attempt to mOdify.<br />

30 CD Cache Disable. If CD=1, no further cache fills occur. However, data already present in the<br />

cache continues to be used if the requested address hits in the cache. The cache must also<br />

be invalidated to completely disable any cache activity.<br />

31 PG Paging Enable Bit. If PG=1 and protected mode is enabled (PE=1), paging is enabled.<br />

2-20 Programming Interface

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