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21264/EV68A Hardware Reference Manual
Privileged Architecture Library Code 6–3
Required PALcode Function Codes
When executing in PALmode, there are certain restrictions for using the privileged
instructions because PALmode gives the programmer complete access to many of the
internal details of the 21264/EV68A. Refer to Section 6.4 for information on these spe-
cial PALmode instructions.
Caution: It is possible to cause unintended side effects by writing what appears to be
perfectly acceptable PALcode. As such, PALcode is not something that
many users will want to change. Before writing PALcode, at least become
familiar with the information in Appendix D.
6.3 Required PALcode Function Codes
Table 6–1 lists opcodes required for all Alpha implementations. The notation used is
oo.ffff, where oo is the hexadecimal 6-bit opcode and ffff is the hexadecimal 26-bit
function code.
6.4 Opcodes Reserved for PALcode
Table 6–2 lists the opcodes reserved by the Alpha architecture for implementation-spe-
cific use. These opcodes are privileged and are only available in PALmode.
These instructions generally produce an OPCDEC exception if executed while the pro-
cessor is not in PALmode. If I_CTL[HWE] is set, these instructions can also be exe-
cuted in kernel mode. Software that uses these instructions must adhere to the PALcode
restrictions listed in this section.
6.4.1 HW_LD Instruction
PALcode uses the HW_LD instruction to access memory outside the realm of normal
Alpha memory management and to perform special Dstream load transactions. Data
alignment traps are disabled for the HW_LD instruction.
Figure 6–1 shows the HW_LD instruction format.
Table 6–1 Required PALcode Function Codes
Mnemonic Type Function Code
DRAINA Privileged 00.0002
HALT Privileged 00.0000
IMB Unprivileged 00.0086
Table 6–2 Opcodes Reserved for PALcode
Mnemonic Opcode
Architecture
Mnemonic Function
HW_LD 1B PAL1B Dstream load instruction
HW_ST 1F PAL1F Dstream store instruction
HW_RET 1E PAL1E Return from PALcode routine
HW_MFPR 19 PAL19 Copies the value of an IPR into an integer GPR
HW_MTPR 1D PAL1D Writes the value of an integer GPR into an IPR