16. Nuclei processor core CSRs Descriptions

16.1. CSR Overview

CSR (Control and Status Registers) is part of RISC-V standard privileged architecture. Basically, Nuclei processor core are following and compatible to RISC-V standard CSR definitions, but there are some additions and enhancements to the original standard spec.

To respect the RISC-V standard, this document may not repeat the contents of original RISC-V standard, but will highlight the additions and enhancements of Nuclei defined.

16.2. Nuclei processor core CSRs List

CSR supported in the Nuclei processor core describes the CSRs in the Nuclei processor core. In this CSRs List, there are RISC-V standard CSRs and customized CSRs in the Nuclei processor core.

Table 16.1 CSR supported in the Nuclei processor core

Type

Address

R & W

Name

Description

RISC-V
Standard
CSRs
These CSRs are following RISC-V standard privileged architecture specification.
This document will not repeat its content here,
please refer to RISC-V standard privileged architecture specification for more details.
Nuclei
Customized
CSRs

0x307

MRW

mtvt

ECLIC Interrupt Vector Table Base Address

Ox345

MRW

mnxti

Used to enable taking the next interrupt and return the entry address of the next interrupt handler.

0x346

MRO

mintstatus

Current Interrupt Levels

0x348

MRW

mscratchcsw

Scratch swap register for privileged mode.

0x349

MRW

mscratchcswl

Scratch swap register for interrupt levels.

0x320

MRW

mcountinhibit

Customized register used to control the on & off of counters

0x7c0

MRW

milm_ctl

Enable/Disable the ILM address space.

0x7c1

MRW

mdlm_ctl

Enable/Disable the DLM address space.

0x7c3

MRO

mnvec

Customized register used to indicate the NMI handler entry address

0x7c4

MRW

msubm

Customized register storing current trap type and the previous trap type before trapped.

0x7c9

MRW

mdcause

Customized register storing current trap’s detailed cause.

0x7ca

MRW

mcache_ctl

Customized register to control the cache features.

0x7d0

MRW

mmisc_ctl

Customized register controlling the selection of the NMI Handler Entry Address.

0x7d6

MRW

msavestatus

Customized register storing the value of mstatus.

0x7d7

MRW

msaveepc1

Customized register storing the value of mepc for the first-level preempted NMI or Exception.

0x7d8

MRW

msavecause1

Customized register storing the value of mcause for the first-level preempted NMI or Exception.

0x7d9

MRW

msaveepc2

Customized register storing the value of mepc for the second-level preempted NMI or Exception.

0x7da

MRW

msavecause2

Customized register storing the value of mcause for the second-level preempted NMI or Exception.

0x7eb

MRW

pushmsubm

Customized register used to push the value of msubm into the stack memory.

0x7ec

MRW

mtvt2

Customized register used to indicate the common handler entry address of non-vectored interrupts.

0x7ed

MRW

jalmnxti

Customized register used to enable the ECLIC interrupt.
The read operation of this register will take the next interrupt, return the entry address of next
interrupt handler, and jump to the corresponding handler at the same time.

0x7ee

MRW

pushmcause

Customized register used to push the value of mcause into the stack memory.

0x7ef

MRW

pushmepc

Customized register used to push the value of mepc into the stack memory.

0x7f0

MRO

mppicfg_info

PPI configuration information.

0x7f1

MRO

mfiocfg_info

FIO configuration information.

0x811

MRW

sleepvalue

Customized register used to indicate the WFI sleep mode.

0x812

MRW

txevt

Customized register used to send an event.

0x810

MRW

wfe

Customized register used to control the WFE mode.

0xfc0

MRO

micfg_info

ILM and I-Cache configuration information.

0xfc1

MRO

mdcfg_info

DLM and D-Cache configuration information.

0xfc2

MRO

mcfg_info

Processor configuration information.

Note

  • MRW – Machine Mode Readable/Writeable

  • MRO – Machine Mode Read-Only

  • URW – User Mode Readable/Writeable

  • URO – User Mode Read-Only

16.3. Accessibility of CSRs in the Nuclei processor core

The CSRs accessibility in the Nuclei processor core:

  • Read or write to a non-existent CSR will raise an Illegal Instruction Exception.

  • Write to RO CSRs will raise an Illegal Instruction Exception.

    Note

    According to the RISC-V standard, the URO registers like cycle, cycleh, time, timeh, instret, instreth are special, the read accessibility of which are determined by the corresponding field in mcounteren.

  • Access the higher privilege mode CSR raise an Illegal Instruction Exception. For example, in User Mode accessing to MRO or MRW CSRs will raise an Illegal Instruction Exception.

16.4. RISC-V Standard CSRs Supported in the Nuclei processor core

These CSRs are following RISC-V standard privileged architecture specification. This document will not repeat its content here, please refer to RISC-V standard privileged architecture specification for more details.

This chapter only introduces the RISC-V Standard CSRs supported in the Nuclei processor core, but those CSRs who also have some unique differences implemented in Nuclei processor core.

16.4.1. mie

In Nuclei processor core, the format and features of CSR mie is same as RISC-V standard, please refer to RISC-V standard privileged architecture specification for more details of CSR mie.

But note: the mie CSR is not effective when the core is in the CLIC mode, and the readout of mie is always 0, the writing is ignored.

16.4.2. mip

In Nuclei processor core, the format and features of CSR mip is same as RISC-V standard, please refer to RISC-V standard privileged architecture specification for more details of CSR mip.

But note: the mip CSR is not effective when the core is in the CLIC mode, and the readout of mip is always 0, the writing is ignored.

16.4.3. mhartid

In Nuclei processor core, the format and features of CSR mhartid is same as RISC-V standard, please refer to RISC-V standard privileged architecture specification for more details of CSR mhartid.

In the Nuclei processor core, hart ID is controlled by input signal core_mhartid.

Note

According to RISC-V architecture, we must ensure exactly one hart runs some code and so require one hart to have a known hart ID of 0.

16.4.4. mtvec

The mtvec register holds the entry address of the interrupt and exception handler. Field of mtvec register is shown in mtvec register.

  • When mtvec holds the exception entry address:

    • The value of the address field must always be aligned on a 4-byte boundary.

  • When mtvec holds the interrupt entry address:

    • When mtvec.MODE != 6’b000011, the processor uses the CLINT interrupt mode.

    • When mtvec.MODE = 6’b000011, the processor uses the CLIC interrupt mode.

Table 16.2 mtvec register

Field

Bit

Description

ADDR

31:6

mtvec address

MODE

5:0

  • MODE field determine interrupt mode:

    • 000011: CLIC interrupt mode

    • Others: CLINT interrupt mode

16.4.5. mcause

The mcause is written with a code indicating the reason that caused the trap. The format and features of CSR mcause is basically same as RISC-V standard, please refer to RISC-V standard privileged architecture specification for more details. But in CLIC mode for Nuclei processor core, there some additional fields added to support CLIC mode interrupt handling.

The mcause register is formatted as shown in mcause register.

Table 16.3 mcause register

Field

Bit

Description

Note

INTERRUPT

31

Current trap type:

  • 0: Exception or NMI

  • 1: Interrupt

MINHV

30

Indicate processer is reading
interrupt vector table
Note: These fields are only effect in CLIC mode.
When in CLINT mode, these field is masked read
as zero, write ignored.

MPP

29:28

privilege mode before interrupt

MPIE

27

Interrupt enable before interrupt

Reserved

26:24

Reserved 0

MPIL

23:16

Previous interrupt level

Reserved

15:12

Reserved 0

EXCCODE

11:0

Exception/Interrupt Encoding

Note

  • In CLIC mode, the mstatus.MPIE and mstatus.MPP are the mirror images of mcause.MPIE and mcause.MPP.

  • The mcause.EXCCODE of NMI can be 0x1 or 0xfff, the value is controlled by mmisc_ctl.

16.4.6. mcycle and mcycleh

The RISC-V architecture defines a 64-bits width cycle counter which indicates how many cycles the processor has executed. Whenever the processor is working, the counter will increase automatically.

The mcycle register records the lower 32-bits of counter and mcycleh records the higher 32-bits. The format and features of CSR mcycle/mcycleh are basically same as RISC-V standard, please refer to RISC-V standard privileged architecture specification for more details.

But in Nuclei processor core, considering the counter increases the power consumption, there is an extra bit in the customized CSR mcountinhibit that can pause the counter to save power when users don’t need to monitor the performance through the counter. See mcountinhibit for more information.

16.4.7. minstret and minstreth

The RISC-V architecture defines a 64-bits width counter which records how many instructions have been executed successfully.

The minstret register records the low 32-bits of counter and minstreth records the high 32-bits. The format and features of CSR minstret/minstreth are basically same as RISC-V standard, please refer to RISC-V standard privileged architecture specification for more details.

But in Nuclei processor core, considering the counter has power consumption, there is an extra bit in the customized CSR mcountinhibit that can turn off the counter to save power when users don’t need to learn the performance through the counter. See mcountinhibit for more information.

16.5. Nuclei processor core Customized CSR

This section introduces customized CSRs in the Nuclei processor core.

16.5.1. mcountinhibit

The mcountinhibit register controls the counting of mcycle/mcycleh and minstret/minstreth registers to save power when users don’t need them. See mcycle and mcycleh and minstret and minstreth for more information.

Table 16.4 mcountinhibit register

Field

Bit

Description

Reserved

31:3

Reserved 0

IR

2

When IR is 1, minstret/minstreth is stop counting

Reserved

1

Reserved 0

CY

0

When CY is 1, mcycle/mcycleh is is stop counting

16.5.2. milm_ctl

The milm_ctl register controls the ILM (Instruction Local Memory) address space to enable or disable it based on user’s application scenarios.

Table 16.5 milm_ctl register

Field

Bit

Description

ILM_BPA

XLEN-1:10
(Read-Only)
The base physical address of ILM. It has to be aligned to multiple of ILM size.
For example, to set up an instruction local memory of size 4KB starting at address 12KB (0x3000),
we simply program ILM_BPA to 0xC (take the upper 22 bits of 0x3000).

Reserved

9:1

Reserved 0

ILM_ENABLE

0
(Writeable in UX class cores)

Instruction Local Memory enable bit.

  • 0: ILM is disabled.

  • 1: ILM is enabled. (default)

Note

ILM can only be disabled in UX class core when MMU and ILM coexist.

16.5.3. mdlm_ctl

The mdlm_ctl register controls the DLM (Data Local Memory) address space to enable or disable it based on user’s application scenarios.

Table 16.6 mdlm_ctl register

Field

Bit

Description

DLM_BPA

XLEN-1:10
(Read-Only)
The base physical address of DLM. It has to be aligned to multiple of DLM size.
For example, to set up an instruction local memory of size 4KB starting at address 12KB (0x3000),
we simply program DLM_BPA to 0xC (take the upper 22 bits of 0x3000).

Reserved

9:1

Reserved 0

DLM_ENABLE

0
(Writeable in UX class cores)

Data Local Memory enable bit.

  • 0: DLM is disabled.

  • 1: DLM is enabled. (default)

Note

DLM can only be disabled in UX class core when MMU and DLM coexist.

16.5.4. mnvec

The Nuclei processor core customized CSR mnvec register holds the NMI entry address.

In order to understand this register, please see NMI Handling in Nuclei processor core for more information.

During a processor running a program, the program will be forced to jump into a new PC address when an NMI is triggered. The PC address is determined by mnvec.

The value of mnvec is controlled by mmisc_ctl.

16.5.5. msubm

The Nuclei processor core customized CSR msubm register holds the current machine sub-mode and the machine sub-mode before the current trap. See Machine Sub-Mode added by Nuclei for more information.

Table 16.7 msubm register

Field

Bit

Description

Reserved

31:10

Reserved 0

PTYP

9:8

Machine sub-mode before entering the trap:

  • 0: Normal machine mode

  • 1: Interrupt handling mode

  • 2: Exception handling mode

  • 3: NMI handling mode

TYP

7:6

Current machine sub-mode:

  • 0: Normal machine mode

  • 1: Interrupt handling mode

  • 2: Exception handling mode

  • 3: NMI handling mode

Reserved

5:0

Reserved 0

16.5.6. mdcause

Since there might be some exceptions share the same mcause.EXCCODE value. To further record the differences, Nuclei processor core customized CSR mdcause register to record the detailed information about the exception.

Table 16.8 mdcause register

Field

Bit

Description

Reserved

31: 2

Reserved 0

mdcause

1:0

Further record the detailed information about the exception.

When mcause.EXCCODE = 1 (Instruction access fault)

  • 0: Reserved

  • 1: PMP permission violation

  • 2: Bus error

  • 3: Reserved

When mcause.EXCCODE = 5 (Load access fault)

  • 0: Reserved

  • 1: PMP permission violation

  • 2: Bus error

  • 3: NICE extended long pipeline instruction return error.

    Note: Although this error ideally is nothing to do with the Load access fault,
    but they just shared the same mcause.EXCCODE to simplify the hardware implementation

When mcause.EXCCODE = 7 (Store/AMO access fault)

  • 0: Reserved

  • 1: PMP permission violation

  • 2: Bus error

  • 3: Reserved

16.5.7. mcache_ctl

Nuclei processor core customized CSR mcache_ctl register to control the I-Cache and D-Cache features.

Table 16.9 mcache_ctl register

Field

Bit

Description

Reserved

31:17

Reserved 0

DC_EN

16

D-Cache Enable:

  • 0: D-Cache disabled, default

  • 1: D-Cache enabled

Reserved

15:2

Reserved 0

IC_SCPD_MOD

1

Scratchpad Mode Enable:

  • 0: I-Cache as the normal mode

  • 1: I-Cache as the Scratchpad mode.

When under this mode, the Data SRAM of I-Cache
will be reused and downgraded to memory mapped SRAM
which can be accessed by instruction fetch and load/store access,
like ILM and DLM, but called as Scratchpad here.
Note: this mode only take effect when IC_EN bit is disabled by software.

IC_EN

0

I-Cache Enable:

  • 0: I-Cache disabled, default

  • 1: I-Cache enabled

Note

Only when the I-Cache is disabled (IC_EN bit as 0) and scratchpad mode is enabled, i.e., mcache_ctl[1:0] is 2’b10, I-Cache really work as the Scratchpad.

16.5.8. mmisc_ctl

The Nuclei processor core customized CSR mmisc_ctl controls the value of mnvec and the mcause value of NMI, unaligned access enable/disable and BPU enable/disable.

Table 16.10 mmisc_ctl register

Field

Bit

Description

Reserved

31:10

Reserved 0

NMI_CAUSE_FFF

9

Control mnvec and mcause.EXCCODE of NMI:

  • 0: The value of mnvec equals the PC address after reset,
    mcause.EXCCODE of NMI is 0x1.
  • 1: The value of mnvec is the same as the value of mtvec,
    mcause.EXCCODE of NMI is 0xfff

UNALGN_ENABLE

6

Enable or Disable misaligned load/store access, if disabled,
accessing misaligned memory locations will trig an Address
Misaligned exception:
  • 0: Disable misaligned load/store access.

  • 1: Enable misaligned load/store access.

Notice: This field only takes effects for load/store
specified in I/F/D Extension, for load/store specified in A
Extension, misaligned accesses always trigger an
Address Misaligned exception.

Reserved

5:4

Reserved 0

BPU_ENABLE

3

Enable or Disable the BPU Unit:

  • 0: Disable the BPU Unit.

  • 1: Enable the BPU Unit.

This bit is hard-wired to 0 if branch prediction unit that
cannot be disabled and enabled are not implemented.

Reserved

8:0

Reserved 0

16.5.9. msavestatus

The msavestatus holds the value of mstatus and msubm which guarantee mstatus and msubm will not be flushed by NMI or exception. The msavestatus has two-stage stack, and supports up to 3-levels NMI/Exception state save. See Nesting of Interrupt, NMI and Exception for more information.

Table 16.11 msavestatus register

Field

Bit

Description

Reserved

31:16

Reserved 0

PTYP2

15:14

The trap type before taking the second-level NMI/Exception.

Reserved

13:11

Reserved 0

MPP2

10:9

The privilege mode before taking the second-level NMI/Exception.

MPIE2

8

The interrupt enable bit before taking the second-level NMI/Exception.

PTYP1

7:6

The trap type before taking the first-level NMI/Exception.

Reserved

5:3

Reserved 0

MPP1

2:1

The privilege mode before taking the second-level NMI/Exception.

MPIE1

0

The interrupt enable bit before taking the second-level NMI/Exception.

16.5.10. msaveepc1 and msaveepc2

msaveepc1 and msaveepc2 are registers of the first-level NMI/Exception status stack and the second-level NMI/Exception status stack, used to save the PC before the first-level NMI/Exception preemption and the second-level NMI/Exception preemption respectively.

  • msaveepc2 <= msaveepc1 <= mepc <= interrupted PC <= NMI/exception PC

Executing the mret instruction, and the value of mcause.INTERRUPT is 0 (Such as NMI or exception), msaveepc1 and msaveepc2 are used to restore the PC through the first and second level NMI/Exception Status Stacks.

  • msaveepc2 => msaveepc1 => mepc => PC

See Nesting of Interrupt, NMI and Exception for more information.

16.5.11. msavecause1 and msavecause2

msavecause1 and msavecause2 are registers of the first-level NMI/Exception status stack and the second-level NMI/Exception status stack, used to save the mcause before the first-level NMI/Exception preemption and the second-level NMI/Exception preemption respectively.

  • msavecause2 <= msavecause1 <= mcause <= NMI/exception cause

Executing the mret instruction, and the value of mcause.INTERRUPT is 0 (Such as NMI or exception), msavecause1 and msavecause2 are used to restore the mcause through the first and second level NMI/Exception Status Stacks.

  • msavecause2 => msavecause1 => mcause

See Nesting of Interrupt, NMI and Exception for more information.

16.5.12. msavedcause1 and msavedcause2

msavedcause1 and msavedcause2 are registers of the first-level NMI/Exception status stack and the second-level NMI/Exception status stack, used to save the mdcause before the first-level NMI/Exception preemption and the second-level NMI/Exception preemption respectively.

  • msavedcause2 <= msavedcause1 <= mdcause <= NMI/exception dcause

Executing the mret instruction, and the value of mcause.INTERRUPT is 0 (Such as NMI or exception), msavedcause1 and msavedcause2 are used to restore the mdcause through the first and second level NMI/Exception Status Stacks.

  • msavedcause2 => msavedcause1 => mdcause

See Nesting of Interrupt, NMI and Exception for more information.

16.5.13. mtvt

This register is from the CLIC draft of RISC-V fast interrupt task group.

The mtvt register holds the base address of interrupt vector table (in CLIC mode), and the base address is aligned at least 64-byte boundary. See (CLIC mode) Interrupt Vector Table for more information.

In order to improve the performance and reduce the gate count, the alignment of the base address in mtvt is determined by the actual number of interrupts, which is shown in the following table.

Table 16.12 mtvt register

Max interrupt number

mtvt alignment

0 to 16

64-byte

17 to 32

128-byte

33 to 64

256-byte

65 to 128

512-byte

129 to 256

1KB

257 to 512

2KB

513 to 1024

4KB

1025 to 2048

8KB

2045 to 4096

16KB

16.5.14. mnxti

This register is from the CLIC draft of RISC-V fast interrupt task group.

The mnxti register(Next Interrupt Handler Address and Interrupt-Enable CSR)can be used by the software to service the next interrupt when it is in the same privilege mode, without incurring the full cost of an interrupt pipeline flush and context save/restore.

The mnxti CSR is designed to be accessed using CSRRSI/CSRRCI instructions, where the value read is the next interrupt handler address and the write back updates the interrupt-enable status.

Note

  • If the next interrupt is not executed in the same privilege mode, the processor will take the next interrupt directly in a nested way, and mnxti only work when the next interrupt is in the same privilege mode.

  • The mnxti CSR instruction is not the same as normal CSR instructions, the return value is different.

    • The return value of mnxti CSR read instruction is shown below:

      • For the following situations, return 0

        • No valid interrupt.

        • The highest priority interrupt is vectored.

      • When the interrupt non-vectored, return the interrupt entry address

    • The mnxti CSR write operation will update following register:

      • mnxti CSR register is a virtual register, i.e., the write operation will actually apply result on mstatus register, i.e., mstatus is the actual RMW(read-modify-write)operation target register.

      • The mcause.EXCCODE field will be updated to the value of the corresponding ID of the taken interrupt.

      • The mintstatus.MIL will be updated to current interrupt level.

16.5.15. mintstatus

This register is from the CLIC draft of RISC-V fast interrupt task group.

The mintstatus register holds the active interrupt level for all the privilege mode.

Table 16.13 minstatus register

Field

Bit

Description

MIL

31:24

The active interrupt level in machine mode

Reserved

23: 8

Reserved 0

UIL

7:0

The active interrupt level in user mode

16.5.16. mtvt2

mtvt2 is used to indicate the entry address of the common base handler shared by all ECLIC non-vectored interrupts. See more information about mtvt2 in Feature and Latency of Non-Vectored Processing Mode.

Table 16.14 mtvt2 register

Field

Bit

Description

CMMON-CODE-ENTRY

31:2

When mtvt2.MTVT2EN=1, this field determines the entry address
of interrupt common code in ECLIC non-vector mode.

Reserved

1

Reserved 0

MTVT2EN

0

mtvt2 enable:

  • 0: the entry address of interrupt common code in ECLIC
    non-vector mode is determined by mtvec
  • 1: the entry address of interrupt common code in ECLIC
    non-vector mode is determined by mtvt2.COMMON-CODE-ENTRY

16.5.17. jalmnxti

The Nuclei processor core customized CSR jalmnxti to reduce the delay for interrupt and accelerates interrupt tail-chaining.

The jalmnxti included all functionality of mnxti, besides it also include enabling the interrupt, handling the next interrupt, jumping to the next interrupt entry address, and jumping to the interrupt handler. So, the jalmnxti can decrease the instruction numbers to speed up the interrupt handling and tail-chaining.

See more information about mtvt2 in Feature and Latency of Non-Vectored Processing Mode.

16.5.18. pushmsubm

The Nuclei processor core customized CSR pushmsubm provides a method to store the value of msubm in memory space which base address is SP with CSR instruction csrrwi.

For example:

csrrwi x0,  PUSHMSUBM,  1

This instruction stores the value of msubm in SP+1*4 address.

16.5.19. pushmcause

The Nuclei processor core customized CSR pushmcause provides a fast method to store the value of mcause in memory space which base address is SP with CSR instruction csrrwi.

See more information about mtvt2 in Feature and Latency of Non-Vectored Processing Mode.

For example:

csrrwi x0,  PUSHMCAUSE,  1

This instruction stores the value of mcause in SP+1*4 address.

16.5.20. pushmepc

The Nuclei processor core customized CSR pushmepc provides a fast method to store the value of mepc in memory space which base address is SP with CSR instruction csrrwi.

See more information about mtvt2 in Feature and Latency of Non-Vectored Processing Mode.

For example:

csrrwi x0,  PUSHMPEC,  1

This instruction stores the value of mepc in SP+1*4 address.

16.5.21. mscratchcsw

This register is from the CLIC draft of RISC-V fast interrupt task group.

The mscratchcsw register is useful to swap the value between the target register and mscratch when privilege mode change.

Using a CSR read instruction to perform mscratchcsw, when the privilege mode is changed after taking an interrupt, following pseudo instruction operations are performed:

 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
csrrw rd, mscratchcsw, rs1

// Pseudocode operation.

if (mcause.mpp!=M-mode) {
    t = rs1; rd = mscratch; mscratch = t;
} else {
    rd = rs1; // mscratch unchanged.
}

// Usual use: csrrw sp, mscratchcsw, sp

When the processor takes an interrupt in a lower privilege mode, the processor enters a higher privilege mode to handle the interrupt and need to store the status of processor into the stack before taking the interrupt. If the processor continues to use SP in low privilege mode, data in the higher privilege mode will be saved in the memory space which is accessible in the lower privilege mode.

RISC-V defines that when the processor is in a lower privilege mode, then data in SP of the higher privilege mode can be stored in mscratch. And in this way, the value of SP can be recovered from mscratch when the processor goes back to the higher privilege mode.

It will cost a lot of cycles to execute the program above using standard instructions, so RISC-V defines mscratchcsw register. After entering an interrupt, the processor executes an mscratchcsw CSR instruction to swap the value between mscratch and SP to recover the value of SP of the higher privilege mode. At the same time, copy the value of SP of the lower privilege to mscratch. Before the mret instruction to exit interrupt, add an mscratchcsw instruction to swap value between mscratch and SP. It will recover the SP value of the lower privilege mode and store the higher privilege mode SP to mscratch again. In this way, only two instructions are needed to solve the stack pointer (SP) switching problem of different privileged modes, which speeds up interrupt processing.

16.5.22. mscratchcswl

This register is from the CLIC draft of RISC-V fast interrupt task group.

The mscratchcswl register is used to exchange the destination register with the value of mscratch to speed up interrupt processing when switching between multiple interrupt levels.

Using the CSR instruction to read the register mscratchcsw, with unchanged privilege mode, the following register operations are performed when there is a switch between the interrupt handler and the application program:

 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
csrrw rd, mscratchcswl, rs1

// Pseudocode operation.

if ( ( mcause.mpil==0) != ( mintstatus.mil == 0) ) {
    t = rs1; rd = mscratch; mscratch = t;
} else {
    rd = rs1; // mscratch unchanged.
}

// Usual use: csrrw sp, mscratchcswl, sp

In the same privilege mode, separating the interrupt handler task from the task space of the application task can increase robustness, reduce space usage, and facilitate system debugging. The interrupt handler has a non-zero interrupt level while the application task has a zero interrupt level. According to this feature, the RISC-V architecture defines the mscratchcswl register. Similar to mscratchcsw, adding a register instruction of mscratchcswl to the beginning and the end of the interrupt service routine enables a fast stack pointer switch between the interrupt handler and the regular application, ensuring the separation of the stack space between the interrupt handler and the regular application.

16.5.23. sleepvalue

The Nuclei processor core customized CSR sleepvalue controls different sleep modes. See Enter the Sleep Mode for more information.

Table 16.15 sleepvalue register

Field

Bit

Description

SLEEPVALUE

0

Control WFI sleep mode: (Reset default value is 0)

  • 0: shallow sleep mode
    (After WFI, it recommends SoC to turn core_clk off)
  • 1: deep sleep mode
    (After WFI, it recommends SoC to turn core_clk
    and core_aon_clk both off)

Reserved

31:1

Reserved 0

16.5.24. txevt

The Nuclei processor core customized CSR txevt controls output events.

Table 16.16 txevt register

Field

Bit

Description

TXEVT

0

Event control: (Reset default value is 0)

  • If this bit is set as 1, Nuclei processor core will trigger
    a single-cycle pulse output signal tx_evt as event signal.
  • This bit will be automatically cleared to 0
    in the next cycle when it becomes 1.
  • No action, when this bit is set as 0.

Reserved

31:1

Reserved 0

16.5.25. wfe

The Nuclei processor core customized CSR wfe Control whether the processor should be awakened by interrupt or event.

See Wait for Event for more information.

Table 16.17 wfe register

Field

Bit

Description

WFE

0

Control whether the processor should be awakened by interrupt or event.

  • 0: The processor should be awakened by interrupt and NMI in sleep mode.

  • 1: The processor should be awakened by event and NMI in sleep mode.

Reset default value is 0.

Reserved

31:1

Reserved 0

16.5.26. ucode

This register is from the P-extension draft of RISC-V P-extension task group.

This register is only existed when the core have been configured to support the P extension. CSR register ucode is used to record if there is overflow happened in DSP instructions.

Table 16.18 ucode register

Field

Bit

Description

OV

0

Record if there is overflow happened in DSP instructions.
If there is overflow then this field OV is set as 1,
software can write 0 to this register to clear it.

Reset default value is 0.

Reserved

31:1

Reserved 0

16.5.27. mcfg_info

This CSR is used to show the processor configuration information.

Table 16.19 mcfg_info register

Field

Bit

Description

Reserved

31:11

Reserved 0

DCACHE

10

Global configuration for D-CACHE support:

  • 0: No D-CACHE support.

  • 1: Has D-CACHE support.

ICACHE

9

Global configuration for I-CACHE support:

  • 0: No I-CACHE support.

  • 1: Has I-CACHE support.

DLM

8

Global configuration for DLM support:

  • 0: No DLM support.

  • 1: Has DLM support.

ILM

7

Global configuration for ILM support:

  • 0: No ILM support.

  • 1: Has ILM support.

NICE

6

Global configuration for NICE support:

  • 0: No NICE support.

  • 1: Has NICE support.

PPI

5

Global configuration for PPI support:

  • 0: No PPI support.

  • 1: Has PPI support.

FIO

4

Global configuration for FIO support:

  • 0: No FIO support.

  • 1: Has FIO support.

PLIC

3

Global configuration for PLIC support:

  • 0: No PLIC support.

  • 1: Has PLIC support.

CLIC

2

Global configuration for CLICsupport:

  • 0: No CLIC support.

  • 1: Has CLIC support.

ECC

1

Global configuration for ECC support:

  • 0: No ECC support.

  • 1: Has ECC support.

TEE

0

Global configuration for TEE support:

  • 0: No TEE support.

  • 1: Has TEE support.

16.5.28. micfg_info

This CSR is used to show the ILM and I-Cache configuration information.

Table 16.20 micfg_info register

Field

Bit

Description

Reserved

XLEN-1:22

Reserved 0

ILM_XONLY

21

Indicates if ILM is execute-only.
If ILM is execute-only, load/store instructions
cannot access the ILM region:
  • 0: ILM is not execute-only.

  • 1: ILM is execute-only.

ILM_SIZE

20:16

Indicates the size of ILM. The size has to be power of 2:

  • 0: 0 Byte

  • 1: 256 Bytes

  • 2: 512 Bytes

  • 3: 1 KiB

  • 4: 2 KiB

  • 5: 4 KiB

  • 6: 8 KiB

  • 7: 16 KiB

  • 8: 32 KiB

  • 9: 64 KiB

  • 10: 128 KiB

  • 11: 256 KiB

  • 12: 512 KiB

  • 13: 1 MiB

  • 14: 2 MiB

  • 15: 4 MiB

  • 16: 8 MiB

  • 17: 16 MiB

  • 18: 32 MiB

  • 19: 64 MiB

  • 20: 128 MiB

  • 20: 256 MiB

  • 21: 512 MiB

  • Others: Reserved

Reserved

15:10

  • Reserved 0

IC_LSIZE

9:7

I-Cache line size:

  • 0: No I-Cache

  • 1: 8 bytes

  • 2: 16 bytes

  • 3: 32 bytes

  • 4: 64 bytes

  • 5: 128 bytes

  • Others: Reserved

IC_WAY

6:4

I-Cache ways:

  • 0: Direct-mapped

  • 1: 2 way

  • 2: 3 way

  • 3: 4 way

  • 4: 5 way

  • 5: 6 way

  • 6: 7 way

  • 7: 8 way

IC_SET

3:0

I-Cache sets per way:

  • 0: 8

  • 1: 16

  • 2: 32

  • 3: 64

  • 4: 128

  • 5: 256

  • 6: 512

  • 7: 1024

  • 8: 2048

  • 9: 4096

  • 10: 8192

  • Others: Reserved

16.5.29. mdcfg_info

This CSR is used to show the DLM and D-Cache configuration information.

Table 16.21 mdcfg_info register

Field

Bit

Description

Reserved

XLEN-1:21

Reserved 0

DLM_SIZE

20:16

Indicates the size of DLM. The size has to be

power of 2:

  • 0: 0 Byte

  • 1: 256 Bytes

  • 2: 512 Bytes

  • 3: 1 KiB

  • 4: 2 KiB

  • 5: 4 KiB

  • 6: 8 KiB

  • 7: 16 KiB

  • 8: 32 KiB

  • 9: 64 KiB

  • 10: 128 KiB

  • 11: 256 KiB

  • 12: 512 KiB

  • 13: 1 MiB

  • 14: 2 MiB

  • 15: 4 MiB

  • 16: 8 MiB

  • 17: 16 MiB

  • 18: 32 MiB

  • 19: 64 MiB

  • 20: 128 MiB

  • 20: 256 MiB

  • 21: 512 MiB

  • Others: Reserved

Reserved

15:10

  • Reserved 0

DC_LSIZE

9:7

D-Cache line size:

  • 0: No D-Cache

  • 1: 8 bytes

  • 2: 16 bytes

  • 3: 32 bytes

  • 4: 64 bytes

  • 5: 128 bytes

  • Others: Reserved

DC_WAY

6:4

D-Cache ways:

  • 0: Direct-mapped

  • 1: 2 way

  • 2: 3 way

  • 3: 4 way

  • 4: 5 way

  • 5: 6 way

  • 6: 7 way

  • 7: 8 way

DC_SET

3:0

D-Cache sets per way:

  • 0: 8

  • 1: 16

  • 2: 32

  • 3: 64

  • 4: 128

  • 5: 256

  • 6: 512

  • 7: 1024

  • 8: 2048

  • 9: 4096

  • 10: 8192

  • Others: Reserved

16.5.30. mppicfg_info

This CSR is used to show the PPI configuration information.

Table 16.22 mppicfg_info register

Field

Bit

Description

PPI_BPA

XLEN-1:10

The base physical address of PPI. It has to be
aligned to multiple of PPI size. For example,
to set up an instruction local memory of size 4KB
starting at address 12KB (0x3000), we simply program DLM_BPA to
0xC (take the upper 22 bits of 0x3000).

Reserved

9:6

Reserved 0

PPI_SIZE

5:1

Indicates the size of PPI. The size has to be power of 2:

  • 0: Reserved

  • 1: 1 KiB

  • 2: 2 KiB

  • 3: 4 KiB

  • 4: 8 KiB

  • 5: 16 KiB

  • 6: 32 KiB

  • 7: 64 KiB

  • 8: 128 KiB

  • 9: 256 KiB

  • 10: 512 KiB

  • 11: 1 MiB

  • 12: 2 MiB

  • 13: 4 MiB

  • 14: 8 MiB

  • 15: 16 MiB

  • 16: 32 MiB

  • 17: 64 MiB

  • 18: 128 MiB

  • 19: 256 MiB

  • 20: 512 MiB

  • 21: 1 GiB

  • 22: 2 GiB

Others: Reserved

Reserved

0

Reserved 1

Note

This CSR exists only when PPI is configured.

16.5.31. mfiocfg_info

This CSR is used to show the FIO configuration information.

Table 16.23 mfiocfg_info register

Field

Bit

Description

FIO_BPA

XLEN-1:10

The base physical address of FIO. It has to be aligned
to multiple of FIO size. For example, to set up an
instruction local memory of size 4KB starting at
address 12KB (0x3000), we simply program FIO_BPA to
0xC (take the upper 22 bits of 0x3000).

Reserved

9:6

Reserved 0

FIO_SIZE

5:1

Indicates the size of FIO. The size has to be

power of 2:

  • 0: Reserved

  • 1: 1 KiB

  • 2: 2 KiB

  • 3: 4 KiB

  • 4: 8 KiB

  • 5: 16 KiB

  • 6: 32 KiB

  • 7: 64 KiB

  • 8: 128 KiB

  • 9: 256 KiB

  • 10: 512 KiB

  • 11: 1 MiB

  • 12: 2 MiB

  • 13: 4 MiB

  • 14: 8 MiB

  • 15: 16 MiB

  • 16: 32 MiB

  • 17: 64 MiB

  • 18: 128 MiB

  • 19: 256 MiB

  • 20: 512 MiB

  • 21: 1 GiB

  • 22: 2 GiB

Others: Reserved

Reserved

0

Reserved 1

Note: this CSR exists only when FIO is configured.