Why in A64 the coprocessor is removed?

Hi,

Even in AArch32 the coprocessors are only conceptual, with the opcode space being used to access system registers (CP15), debug registers (CP14), and FP / NEON (CP10 & CP11). This is contrast to older versions of the architecture where there needed to be a way to access actual coprocessors in the system.

In AArch64, the conceptual coprocessors have been removed and we now only access system registers using MSR/MRS instructions with the target register's name.

Take for example CPACR_EL1, which is architecturally mapped to the AArch32 register CPACR. To access CPACR in AArch32 we need to use coprocessor instructions:

MRC p15,0,<Rt>,c1,c0,2 ; Read CPACR into Rt

MCR p15,0,<Rt>,c1,c0,2 ; Write Rt to CPACR

Whereas to access CPACR_EL1 in AArch64 we just use the register name:

MRS <Xt>, CPACR_EL1 ; Read CPACR_EL1 into Xt

MSR CPACR_EL1, <Xt> ; Write Xt to CPACR_EL1

Hope that helps,

Ash.

Hi,

Even in AArch32 the coprocessors are only conceptual, with the opcode space being used to access system registers (CP15), debug registers (CP14), and FP / NEON (CP10 & CP11). This is contrast to older versions of the architecture where there needed to be a way to access actual coprocessors in the system.

In AArch64, the conceptual coprocessors have been removed and we now only access system registers using MSR/MRS instructions with the target register's name.

Take for example CPACR_EL1, which is architecturally mapped to the AArch32 register CPACR. To access CPACR in AArch32 we need to use coprocessor instructions:

MRC p15,0,<Rt>,c1,c0,2 ; Read CPACR into Rt

MCR p15,0,<Rt>,c1,c0,2 ; Write Rt to CPACR

Whereas to access CPACR_EL1 in AArch64 we just use the register name:

MRS <Xt>, CPACR_EL1 ; Read CPACR_EL1 into Xt

MSR CPACR_EL1, <Xt> ; Write Xt to CPACR_EL1

Hope that helps,

Ash.