Translation Buffer (TB) Fill Flows

hw_mtprp4, <EV6__DTB_PTE0 ! ^x44>

; (0,4,2,6) (0L) write pte0

hw_mtprp4, <EV6__DTB_PTE1 ! ^x22>

; (3,7,1,5) (1L) write pte1

ASSUME <tb_mb_en + pte_eco> ne 2

 

.if ne pte_eco

 

bnep7, trap__dtbm_single_mb

; branch for mb

hw_ret (p23)

; return

trap__dtbm_single_mb:

 

mb

 

hw_ret(p23)

; return

.iff

 

hw_ret(p23)

; return

 

; (assumes tb_mb_en on multi-processors)

.endc

 

The following list presents information about the single-miss DTB code example:

In Figure 6–5,where (x,y) or (y) appear in the comments, x specifies the scoreboard bits and y specifies the Ebox subcluster.

r4 r7 and r20 r23 are PALshadow registers.

PALshadow r22 contains a flag that indicates whether the native code is running “1 to1”, that is, running in a mode where the physical address should be mapped 1to1 to the virtual address, rather than being taken from a page table.

IPR scoreboard bits [3:0] are used to order the restarted load or store instructions for the DTB write transactions.

MM_STAT and VA will not be overwritten if the LD_VPTE instruction misses the DTB. There is no issue order constraint.

The code is written to prevent a later execution of the DTB fill instruction from being issued before a previous execution and corrupting the previous write to the TB registers. The correct sequence of executions is accomplished by placing code dependencies on scoreboard bits [7:4] in the path of the successive writers. This prevents the successive writers from being issued before the previous writers are retired.

When I_CTL[TB_MB_EN] = 1, the issue of MTPR DTB_PTE0 triggers, in hard- ware, a lightweight memory barrier (TB-MB). The lightweight memory barrier enforces read-ordering of store instructions from another processor (I) to this pro- cessor’s (J) page table and this processor’s virtual memory area such that if this processor sees the write to the PTE from (I) it will see the new data.

Processor I

Processor J

 

 

Wr Data

LD/ST

MB

<tb miss>

Wr PTE

LD-PTE, write TB

 

LD/ST

 

 

Alpha 21264/EV67 Hardware Reference Manual

Privileged Architecture Library Code 6–15

Page 199
Image 199
Compaq 21264, EV67 specifications Wr Data, Tb miss, Wr PTE LD-PTE, write TB