Clock Control
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AMD-K5 Processor Technical Reference Manual
state, and it returns to the Halt state when STPCLK is negated.
No processor registers are saved before entering the Halt state
because the processor returns to the next unexecuted instruc-
tion in program order when it returns to its prior operating
mode. When the processor returns to the Halt state, the HLT
instruction is not refetched but the processor drives the Halt
special bus cycle on the bus after the return.
Within the Halt state, the processor disables the majority of its
internal clock distribution and (if STPCLK is asserted) the
internal pullup resistor on STPCLK. However, its phase-lock
loop still runs, its key internal logic is still clocked, most of its
inputs and outputs retain their last state (except D63–D0 and
DP7–DP0 which are floated), and it still responds to input sig-
nals.
The HLT instruction is commonly executed by modern UNIX-
type operating systems as a method of entering an idle loop.
The operating system sees that it has no pending processes,
therefore nothing to execute, so it executes HLT. Entry into
the Halt state achieves the same power-saving effect as entry
into the Stop Grant state, but the method is simpler and faster.
Entry into the Halt state requires only the execution of the
HLT instruction, whereas entry into the Stop Grant state
requires that system logic monitor system activity, assert STP-
CLK, and decode the processor’s acknowledgment (potentially
several clocks later) via the Stop Grant special bus cycle.
Summary of Contents for AMD-K5
Page 1: ...AMD K5 Processor Technical Reference Manual TM...
Page 10: ...x AMD K5 Processor Technical Reference Manual 18524C 0 Nov1996...
Page 24: ...1 4 Overview AMD K5 Processor Technical Reference Manual 18524C 0 Nov1996...
Page 54: ...2 30 Internal Architecture AMD K5 Processor Technical Reference Manual 18524C 0 Nov1996...
Page 116: ...4 26 Performance AMD K5 Processor Technical Reference Manual 18524C 0 Nov1996...
Page 356: ...6 44 System Design AMD K5 Processor Technical Reference Manual 18524C 0 Nov1996...
Page 380: ...7 24 Test and Debug AMD K5 Processor Technical Reference Manual 18524C 0 Nov1996...
Page 396: ...A 16 AMD K5 Processor Technical Reference Manual 18524C 0 Nov1996...
Page 406: ...I 10 Index AMD K5 Processor Technical Reference Manual 18524C 0 Nov1996...