Compaq 21264 - Manual (Page 193)

Alpha 21264/EV67 Hardware Reference Manual Privileged Architecture Library Code 6–9
Internal Processor Register Access Mechanisms
6.5.3 Hardware Structure of Implicitly Written IPRs
Implicitly written IPRs are physically built using only a single level of register, how-
ever the IPR has two hardware states associated with it:
1. Default State—The contents of the register may be written when an instruction gen-
erates an exception. If an exception occurs, write a new value into the IPR and go to
state 2.
2. Locked State—The contents of the register may only be overwritten by an except-
ing instruction that is older than the instruction associated with the contents of the
IPR. If such an exception occurs, overwrite the value of the IPR. When the trigger-
ing instruction, or instruction that is older than the triggering instruction, is killed
by the Ibox, go to state 1.
6.5.4 IPR Access Ordering
IPR access mechanisms must allow values to be passed through each IPR from a pro-
ducer to its intended consumers.
Table 6–7 lists all of the paired instruction orderings between instructions of the four
IPR access types. It specifies whether access order must be maintained, and if so, the
mechanisms used to ensure correct ordering.
Table 6–7 Paired Instruction Fetch Order
Second
Instruction
First Instruction
Implicit Reader Implicit Writer Explicit Reader Explicit Writer
Implicit
Reader Read transac-
tions can be
reordered. No IPRs in this class. Read transactions can
be reordered.
A variety of mechanisms are
used to ensure order:
scoreboard bits to stall issue of
reader; HW_RET_STALL to
stall reader; double write plus
buffer blocks to force retire and
allow for propagation delay.
Implicit
Writer No IPRs in this
class. The hardware struc-
ture of implicitly
written IPRs handles
this case. IPR-specific PALcode
restrictions are
required for this case.
An interlock mecha-
nism must be placed
between the explicit
reader and the implicit
writer (a read transac-
tion). No IPRs in this class.
Explicit
Reader Read transac-
tions can be
reordered. If the reader is in the
PALcode routine
invoked by the
exception associated
with the writer, then
ordering is guaran-
teed. Read transactions can
be reordered. Scoreboard bits stall issue of
reader until writer is retired.