Compaq 21264, EV67 specifications Hwret Instruction, 4describes the Hwst instruction fields

Opcodes Reserved for PALcode

Table 6–4describes the HW_ST instruction fields.

Table 6–4 HW_ST Instruction Fields Descriptions

Extent	Mnemonic	Value	Description
[31:26]	OPCODE	1F16	The opcode value.
[25:21]	RA	—	Write data register number.
[20:16]	RB	—	Base register for memory address.
[15:13]	TYPE	0002	Physical — The effective address for the HW_ST instruction is physical.
		0012	Physical/Cond — The effective address for the HW_ST instruction is
			physical. Store conditional version of the HW_ST instruction. The lock
			flag is returned in RA. Refer to PALcode restrictions for correct use of this
			function.
		0102	Virtual — The effective address for the HW_ST instruction is virtual.
		1102	Virtual/Alt — The effective address for the HW_ST instruction is virtual.
			Access checks use DTB_ ALT_MODE IPR.
		All others	Unused.
[12]	LEN	0	Access length is longword.
		1	Access length is quadword.
[11:0]	DISP	—	Holds a 12-bit signed byte displacement.

6.4.3 HW_RET Instruction

The HW_RET instruction is used to return instruction flow to a specified PC. The RB field of the HW_RET instruction specifies an integer GPR, which holds the new value of the PC. Bit [0] of this register provides the new value of PALmode after the HW_RET instruction is executed. Bits [15:14] of the instruction determine the stack action.

Normally the HW_RET instruction succeeds a CALL_PAL instruction, or a trap han- dler that pushed its PC onto the prediction stack. In this mode, the HINT should be set to ‘10’ to pop the PC and generate a predicted target address for the HW_RET instruc- tion.

In some conditions, the HW_RET instruction is used in the middle of a PALcode flow to cause a group of instructions to retire. In these cases, if the HW_RET instruction does not have a corresponding instruction that pushed a PC onto the stack, the HINT field should be set to ‘00’ to keep the stack from being modified.

In the rare circumstance that the HW_RET instruction might be used like a JSR or JSR_COROUTINE, the stack can be managed by setting the HINT bits accordingly.

See Section D.25 for more information about the HW_RET instruction.

Figure 6–3shows the HW_RET instruction format.

Alpha 21264/EV67 Hardware Reference Manual Privileged Architecture Library Code 6–5