Front-end to back-end interface

The front-end is responsible for fetching instructions and the back-end is responsible for executing them. This interface ensures that the back-end has instructions to execute and that the front-end is aware of any control flow in the program.

Front-end to back-end

Signal	Type	Description
fetch_valid	Boolean	Whether fetch_instr is valid.
fetch_instr	fetched_instr_t	The instruction to execute. (More details)

Back-end to front-end

Signal	Type	Description
satp	64 bits	The value of the SATP (Supervisor Address Translation and Protection) control register.
prv	priv_lvl_e	The current privilege level. (Available options)
status	status_t	A collection of relevant information from the control registers. (More details)
redirect_valid	Boolean	Whether a redirection (e.g. branch, jump) is currently being requested.
redirect_reason	if_reason_e	The reason for the requested redirection. (Available options)
redirect_pc	64 bits	The new memory address to begin fetching instructions from.
branch_info	branch_info_t	Non-speculative information about executed branches, used (for example) to update the branch predictor. (More details)
fetch_ready	Boolean	Whether the back-end is ready to receive a new instruction.

fetched_instr_t

A fetched instruction and relevant metadata. A compound type consisting of:

Field	Type	Description
pc	64 bits	Program counter.
if_reason	if_reason_e	Reason why the instruction was fetched (i.e. was it speculative?). (Available options)
instr_word	32 bits	The instruction that was fetched.
ex_valid	Boolean	Whether an exception occurred during instruction fetch.
exception	exception_t	Further information on the exception. (More details)

if_reason_e

Reasons for an instruction fetch taking place.

Value	Description
IF_PREFETCH	An instruction prefetch that follows the previous instruction in program counter order.
IF_PREDICT	An instruction prefetch commanded by the branch predictor.
IF_MISPREDICT	An instruction fetch caused by misprediction.
IF_PROT_CHANGED	Memory protection bits, e.g. MSTATUS, PRV or SATP has been changed.
IF_SATP_CHANGED	The SATP (Supervisor Address Translation and Protection) control register has been changed.
IF_FENCE_I	The FENCE.I instruction was executed.
IF_SFENCE_VMA	The SFENCE.VMA instruction was executed.

exception_t

Information about an exception. A compound type consisting of:

Field	Type	Description
cause	exc_cause_e	The cause of the exception. The only possible instruction fetch exceptions in Muntjac are EXC_CAUSE_INSTR_PAGE_FAULT and EXC_CAUSE_INSTR_ACCESS_FAULT.
tval	64 bits	Address being accessed when the exception occurred.

priv_lvl_e

Privilege levels.

Value	Description
PRIV_LVL_M	Machine mode.
PRIV_LVL_H	Hypervisor mode.
PRIV_LVL_S	Supervisor mode.
PRIV_LVL_U	User mode.

status_t

A collection of information from the control registers consisting of:

Field	Type	Description
tsr	Boolean	The MSTATUS TSR bit (Trap Supervisor Return).
tw	Boolean	The MSTATUS TW bit (Timeout Wait).
tvm	Boolean	The MSTATUS TVM bit (Trap Virtual Memory).
mxr	Boolean	The MSTATUS MXR bit (Make eXecutable Readable).
sum	Boolean	The MSTATUS SUM bit (permit Supervisor User Memory access).
mprv	Boolean	The MSTATUS MPRV bit (Modify PRiVilege).
fs	2 bits	The MSTATUS FS bits (Floating-point State).
mpp	priv_lvl_e	The MSTATUS MPP bits (Machine mode Previous Privilege).
spp	Boolean	The MSTATUS SPP bit (Supervisor mode Previous Privilege).
mpie	Boolean	The MSTATUS MPIE bit (Machine mode Previous Interrupt-Enable).
spie	Boolean	The MSTATUS SPIE bit (Supervisor mode Previous Interrupt-Enable).
mie	Boolean	The MSTATUS MIE bit (Machine mode Interrupt-Enable).
sie	Boolean	The MSTATUS SIE bit (Supervisor mode Interrupt-Enable).

branch_info_t

Information about a branch operation. A compound type consisting of:

Field	Type	Description
branch_type	branch_type_e	The type of branch. (Available options)
pc	64 bits	The address of the branch instruction (not the address being branched to).
compressed	Boolean	Whether the branch instruction is compressed.

branch_type_e

Types of branch operation.

Value	Description
BRANCH_NONE	Not a branch instruction.
BRANCH_UNTAKEN	Branch not taken.
BRANCH_TAKEN	Branch taken.
BRANCH_JUMP	Unconditional jump.
BRANCH_CALL	Environment call: request additional privilege.
BRANCH_RET	Environment return: return to previous privilege.
BRANCH_YIELD	Voluntarily yield control of the core and trigger a thread switch.