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Alpha 21264/EV67 Hardware Reference Manual
PALcode Restrictions and Guidelines D–17
Restriction 31 : I_CTL[VA_48] Update
sys__cbox_over6: ; block 6
beq p6, sys__cbox_over8 ; branch if done
bis r31, r31, r31 ; nop
br r31, sys__cbox_over7 ; go to block 7
sys__cbox_touch6: ;
br r31, sys__cbox_touch7 ; touch block 7
sys__cbox_over7: ; block 7
bis p7, r31, p20 ; save before shifting
sll p7, #6, p7 ; shift data 6 bits left
br r31, sys__cbox_over2 ; do next shift
sys__cbox_touch7: ;
br r31, sys__cbox_touch8 ; touch block 8
sys__cbox_over8: ; block 8
beq r31, sys__cbox_cbox_done ; predict not taken
PVC_VIOLATE <1006>
br r31, .-4 ; predict back to infinite loop
bis r31, r31, r31 ;
sys__cbox_touch8: ;
br r31, sys__cbox_over1 ; now start executing the shifts
sys__cbox_cbox_done: ; now restore i_ctl
hw_mfpr p6, EV6__I_CTL ; (4,0L) get i_ctl
lda p4, <3@EV6__I_CTL__SBE__S>(r31) ; sbe bits
or p6, p4, p4 ; set SBE bits
bis r31, r31, r31
hw_mtpr p4, EV6__I_CTL ; (4,0L) restore i_ctl
PVC_JSR cbox, bsr=1, dest=1
hw_ret_stall (p5) ; return to caller with stall
D.27 Restriction 31 : I_CTL[VA_48] Update
The VA_48 virtual address format cannot be changed while executing a JSR, JMP,
GOTO, JSR_COROUTINE, or HW_RET instruction. A simple method of ensuring
that the address does not change is to write I_CTL twice, in two separate fetch blocks,
with the same data. The second write will stall the pipeline and ensure that the mode
cannot change, even down a mispredicted path, while a following JSR type instruction
might be using the address comparison logic.
D.28 Restriction 32 : PCTR_CTL Update
The performance counter must not be left in a state near overflow. If counting is dis-
abled, the counters may produce multiple overflow signals if the counter output is not
updated due to the counter being disabled. A repeated overflow signal with counters
disabled can block other incoming interrupt requests while the overflow state persists.
To avoid this situation, reads or writes to the counters should not leave a value near
overflow. In normal operation, with counters enabled, a counter overflow will produce
an overflow pulse, clear the counter, and produce a performance counter interrupt.
Interrupts can only be blocked for one cycle.