GCR_EL1, Tag Control Register.
The GCR_EL1 characteristics are:
Purpose
Tag Control Register.
Configuration
This register is present only when ARMv8.5-MemTag is implemented. Otherwise, direct accesses to GCR_EL1 are UNDEFINED.
RW fields in this register reset to architecturally UNKNOWN values.
Attributes
GCR_EL1 is a 64-bit register.
Field descriptions
The GCR_EL1 bit assignments are:
| 63 | 62 | 61 | 60 | 59 | 58 | 57 | 56 | 55 | 54 | 53 | 52 | 51 | 50 | 49 | 48 | 47 | 46 | 45 | 44 | 43 | 42 | 41 | 40 | 39 | 38 | 37 | 36 | 35 | 34 | 33 | 32 |
| 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 |
| 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | RRND | Exclude | |||||||||||||||
| 31 | 30 | 29 | 28 | 27 | 26 | 25 | 24 | 23 | 22 | 21 | 20 | 19 | 18 | 17 | 16 | 15 | 14 | 13 | 12 | 11 | 10 | 9 | 8 | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
Bits [63:17]
Reserved, RES0.
RRND, bit [16]
Controls whether RandomTag() generates a deterministic value solely based on the contents of RGSR_EL1, or a non-deterministic value.
| RRND | Meaning |
|---|---|
| 0b0 |
Generate a deterministic value based on RGSR_EL1. |
| 0b1 |
Generate an IMPLEMENTATION DEFINED non-deterministic value. |
When the value of GCR_EL1.RRND is 1, the value generated does not need to be cryptographically random.
A similar algorithm to that used when RRND=0 but with free running clock is sufficient.
This field resets to an architecturally UNKNOWN value.
Exclude, bits [15:0]
Allocation Tag values excluded from selection by ChooseNonExcludedTag().
This field resets to an architecturally UNKNOWN value.
Accessing the GCR_EL1
Accesses to this register use the following encodings:
MRS <Xt>, GCR_EL1
| op0 | CRn | op1 | op2 | CRm |
|---|---|---|---|---|
| 0b11 | 0b0001 | 0b000 | 0b110 | 0b0000 |
if PSTATE.EL == EL0 then
UNDEFINED;
elsif PSTATE.EL == EL1 then
return GCR_EL1;
elsif PSTATE.EL == EL2 then
return GCR_EL1;
elsif PSTATE.EL == EL3 then
return GCR_EL1;
MSR GCR_EL1, <Xt>
| op0 | CRn | op1 | op2 | CRm |
|---|---|---|---|---|
| 0b11 | 0b0001 | 0b000 | 0b110 | 0b0000 |
if PSTATE.EL == EL0 then
UNDEFINED;
elsif PSTATE.EL == EL1 then
GCR_EL1 = X[t];
elsif PSTATE.EL == EL2 then
GCR_EL1 = X[t];
elsif PSTATE.EL == EL3 then
GCR_EL1 = X[t];