1. Introduction to LoongArch

LoongArch is a new RISC ISA, which is a bit like MIPS or RISC-V. There are currently 3 variants: a reduced 32-bit version (LA32R), a standard 32-bit version (LA32S) and a 64-bit version (LA64). There are 4 privilege levels (PLVs) defined in LoongArch: PLV0~PLV3, from high to low. Kernel runs at PLV0 while applications run at PLV3. This document introduces the registers, basic instruction set, virtual memory and some other topics of LoongArch.

1.1. Registers

LoongArch registers include general purpose registers (GPRs), floating point registers (FPRs), vector registers (VRs) and control status registers (CSRs) used in privileged mode (PLV0).

1.1.1. GPRs

LoongArch has 32 GPRs ( $r0 ~ $r31 ); each one is 32-bit wide in LA32 and 64-bit wide in LA64. $r0 is hard-wired to zero, and the other registers are not architecturally special. (Except $r1, which is hard-wired as the link register of the BL instruction.)

The kernel uses a variant of the LoongArch register convention, as described in the LoongArch ELF psABI spec, in References:

Name

Alias

Usage

Preserved across calls

$r0

$zero

Constant zero

Unused

$r1

$ra

Return address

No

$r2

$tp

TLS/Thread pointer

Unused

$r3

$sp

Stack pointer

Yes

$r4-$r11

$a0-$a7

Argument registers

No

$r4-$r5

$v0-$v1

Return value

No

$r12-$r20

$t0-$t8

Temp registers

No

$r21

$u0

Percpu base address

Unused

$r22

$fp

Frame pointer

Yes

$r23-$r31

$s0-$s8

Static registers

Yes

Note

The register $r21 is reserved in the ELF psABI, but used by the Linux kernel for storing the percpu base address. It normally has no ABI name, but is called $u0 in the kernel. You may also see $v0 or $v1 in some old code,however they are deprecated aliases of $a0 and $a1 respectively.

1.1.2. FPRs

LoongArch has 32 FPRs ( $f0 ~ $f31 ) when FPU is present. Each one is 64-bit wide on the LA64 cores.

The floating-point register convention is the same as described in the LoongArch ELF psABI spec:

Name

Alias

Usage

Preserved across calls

$f0-$f7

$fa0-$fa7

Argument registers

No

$f0-$f1

$fv0-$fv1

Return value

No

$f8-$f23

$ft0-$ft15

Temp registers

No

$f24-$f31

$fs0-$fs7

Static registers

Yes

Note

You may see $fv0 or $fv1 in some old code, however they are deprecated aliases of $fa0 and $fa1 respectively.

1.1.3. VRs

There are currently 2 vector extensions to LoongArch:

  • LSX (Loongson SIMD eXtension) with 128-bit vectors,

  • LASX (Loongson Advanced SIMD eXtension) with 256-bit vectors.

LSX brings $v0 ~ $v31 while LASX brings $x0 ~ $x31 as the vector registers.

The VRs overlap with FPRs: for example, on a core implementing LSX and LASX, the lower 128 bits of $x0 is shared with $v0, and the lower 64 bits of $v0 is shared with $f0; same with all other VRs.

1.1.4. CSRs

CSRs can only be accessed from privileged mode (PLV0):

Address

Full Name

Abbrev Name

0x0

Current Mode Information

CRMD

0x1

Pre-exception Mode Information

PRMD

0x2

Extension Unit Enable

EUEN

0x3

Miscellaneous Control

MISC

0x4

Exception Configuration

ECFG

0x5

Exception Status

ESTAT

0x6

Exception Return Address

ERA

0x7

Bad (Faulting) Virtual Address

BADV

0x8

Bad (Faulting) Instruction Word

BADI

0xC

Exception Entrypoint Address

EENTRY

0x10

TLB Index

TLBIDX

0x11

TLB Entry High-order Bits

TLBEHI

0x12

TLB Entry Low-order Bits 0

TLBELO0

0x13

TLB Entry Low-order Bits 1

TLBELO1

0x18

Address Space Identifier

ASID

0x19

Page Global Directory Address for Lower-half Address Space

PGDL

0x1A

Page Global Directory Address for Higher-half Address Space

PGDH

0x1B

Page Global Directory Address

PGD

0x1C

Page Walk Control for Lower- half Address Space

PWCL

0x1D

Page Walk Control for Higher- half Address Space

PWCH

0x1E

STLB Page Size

STLBPS

0x1F

Reduced Virtual Address Configuration

RVACFG

0x20

CPU Identifier

CPUID

0x21

Privileged Resource Configuration 1

PRCFG1

0x22

Privileged Resource Configuration 2

PRCFG2

0x23

Privileged Resource Configuration 3

PRCFG3

0x30+n (0≤n≤15)

Saved Data register

SAVEn

0x40

Timer Identifier

TID

0x41

Timer Configuration

TCFG

0x42

Timer Value

TVAL

0x43

Compensation of Timer Count

CNTC

0x44

Timer Interrupt Clearing

TICLR

0x60

LLBit Control

LLBCTL

0x80

Implementation-specific Control 1

IMPCTL1

0x81

Implementation-specific Control 2

IMPCTL2

0x88

TLB Refill Exception Entrypoint Address

TLBRENTRY

0x89

TLB Refill Exception BAD (Faulting) Virtual Address

TLBRBADV

0x8A

TLB Refill Exception Return Address

TLBRERA

0x8B

TLB Refill Exception Saved Data Register

TLBRSAVE

0x8C

TLB Refill Exception Entry Low-order Bits 0

TLBRELO0

0x8D

TLB Refill Exception Entry Low-order Bits 1

TLBRELO1

0x8E

TLB Refill Exception Entry High-order Bits

TLBEHI

0x8F

TLB Refill Exception Pre-exception Mode Information

TLBRPRMD

0x90

Machine Error Control

MERRCTL

0x91

Machine Error Information 1

MERRINFO1

0x92

Machine Error Information 2

MERRINFO2

0x93

Machine Error Exception Entrypoint Address

MERRENTRY

0x94

Machine Error Exception Return Address

MERRERA

0x95

Machine Error Exception Saved Data Register

MERRSAVE

0x98

Cache TAGs

CTAG

0x180+n (0≤n≤3)

Direct Mapping Configuration Window n

DMWn

0x200+2n (0≤n≤31)

Performance Monitor Configuration n

PMCFGn

0x201+2n (0≤n≤31)

Performance Monitor Overall Counter n

PMCNTn

0x300

Memory Load/Store WatchPoint Overall Control

MWPC

0x301

Memory Load/Store WatchPoint Overall Status

MWPS

0x310+8n (0≤n≤7)

Memory Load/Store WatchPoint n Configuration 1

MWPnCFG1

0x311+8n (0≤n≤7)

Memory Load/Store WatchPoint n Configuration 2

MWPnCFG2

0x312+8n (0≤n≤7)

Memory Load/Store WatchPoint n Configuration 3

MWPnCFG3

0x313+8n (0≤n≤7)

Memory Load/Store WatchPoint n Configuration 4

MWPnCFG4

0x380

Instruction Fetch WatchPoint Overall Control

FWPC

0x381

Instruction Fetch WatchPoint Overall Status

FWPS

0x390+8n (0≤n≤7)

Instruction Fetch WatchPoint n Configuration 1

FWPnCFG1

0x391+8n (0≤n≤7)

Instruction Fetch WatchPoint n Configuration 2

FWPnCFG2

0x392+8n (0≤n≤7)

Instruction Fetch WatchPoint n Configuration 3

FWPnCFG3

0x393+8n (0≤n≤7)

Instruction Fetch WatchPoint n Configuration 4

FWPnCFG4

0x500

Debug Register

DBG

0x501

Debug Exception Return Address

DERA

0x502

Debug Exception Saved Data Register

DSAVE

ERA, TLBRERA, MERRERA and DERA are sometimes also known as EPC, TLBREPC, MERREPC and DEPC respectively.

1.2. Basic Instruction Set

1.2.1. Instruction formats

LoongArch instructions are 32 bits wide, belonging to 9 basic instruction formats (and variants of them):

Format name

Composition

2R

Opcode + Rj + Rd

3R

Opcode + Rk + Rj + Rd

4R

Opcode + Ra + Rk + Rj + Rd

2RI8

Opcode + I8 + Rj + Rd

2RI12

Opcode + I12 + Rj + Rd

2RI14

Opcode + I14 + Rj + Rd

2RI16

Opcode + I16 + Rj + Rd

1RI21

Opcode + I21L + Rj + I21H

I26

Opcode + I26L + I26H

Rd is the destination register operand, while Rj, Rk and Ra ("a" stands for "additional") are the source register operands. I8/I12/I14/I16/I21/I26 are immediate operands of respective width. The longer I21 and I26 are stored in separate higher and lower parts in the instruction word, denoted by the "L" and "H" suffixes.

1.2.2. List of Instructions

For brevity, only instruction names (mnemonics) are listed here; please see the References for details.

  1. Arithmetic Instructions:

    ADD.W SUB.W ADDI.W ADD.D SUB.D ADDI.D
SLT SLTU SLTI SLTUI
AND OR NOR XOR ANDN ORN ANDI ORI XORI
MUL.W MULH.W MULH.WU DIV.W DIV.WU MOD.W MOD.WU
MUL.D MULH.D MULH.DU DIV.D DIV.DU MOD.D MOD.DU
PCADDI PCADDU12I PCADDU18I
LU12I.W LU32I.D LU52I.D ADDU16I.D

  2. Bit-shift Instructions:

    SLL.W SRL.W SRA.W ROTR.W SLLI.W SRLI.W SRAI.W ROTRI.W
SLL.D SRL.D SRA.D ROTR.D SLLI.D SRLI.D SRAI.D ROTRI.D

  3. Bit-manipulation Instructions:

    EXT.W.B EXT.W.H CLO.W CLO.D SLZ.W CLZ.D CTO.W CTO.D CTZ.W CTZ.D
BYTEPICK.W BYTEPICK.D BSTRINS.W BSTRINS.D BSTRPICK.W BSTRPICK.D
REVB.2H REVB.4H REVB.2W REVB.D REVH.2W REVH.D BITREV.4B BITREV.8B BITREV.W BITREV.D
MASKEQZ MASKNEZ

  4. Branch Instructions:

    BEQ BNE BLT BGE BLTU BGEU BEQZ BNEZ B BL JIRL

  5. Load/Store Instructions:

    LD.B LD.BU LD.H LD.HU LD.W LD.WU LD.D ST.B ST.H ST.W ST.D
LDX.B LDX.BU LDX.H LDX.HU LDX.W LDX.WU LDX.D STX.B STX.H STX.W STX.D
LDPTR.W LDPTR.D STPTR.W STPTR.D
PRELD PRELDX

  6. Atomic Operation Instructions:

    LL.W SC.W LL.D SC.D
AMSWAP.W AMSWAP.D AMADD.W AMADD.D AMAND.W AMAND.D AMOR.W AMOR.D AMXOR.W AMXOR.D
AMMAX.W AMMAX.D AMMIN.W AMMIN.D

  7. Barrier Instructions:

    IBAR DBAR

  8. Special Instructions:

    SYSCALL BREAK CPUCFG NOP IDLE ERTN(ERET) DBCL(DBGCALL) RDTIMEL.W RDTIMEH.W RDTIME.D
ASRTLE.D ASRTGT.D

  9. Privileged Instructions:

    CSRRD CSRWR CSRXCHG
IOCSRRD.B IOCSRRD.H IOCSRRD.W IOCSRRD.D IOCSRWR.B IOCSRWR.H IOCSRWR.W IOCSRWR.D
CACOP TLBP(TLBSRCH) TLBRD TLBWR TLBFILL TLBCLR TLBFLUSH INVTLB LDDIR LDPTE

1.3. Virtual Memory

LoongArch supports direct-mapped virtual memory and page-mapped virtual memory.

Direct-mapped virtual memory is configured by CSR.DMWn (n=0~3), it has a simple relationship between virtual address (VA) and physical address (PA):

VA = PA + FixedOffset

Page-mapped virtual memory has arbitrary relationship between VA and PA, which is recorded in TLB and page tables. LoongArch's TLB includes a fully-associative MTLB (Multiple Page Size TLB) and set-associative STLB (Single Page Size TLB).

By default, the whole virtual address space of LA32 is configured like this:

Name

Address Range

Attributes

UVRANGE

0x00000000 - 0x7FFFFFFF

Page-mapped, Cached, PLV0~3

KPRANGE0

0x80000000 - 0x9FFFFFFF

Direct-mapped, Uncached, PLV0

KPRANGE1

0xA0000000 - 0xBFFFFFFF

Direct-mapped, Cached, PLV0

KVRANGE

0xC0000000 - 0xFFFFFFFF

Page-mapped, Cached, PLV0

User mode (PLV3) can only access UVRANGE. For direct-mapped KPRANGE0 and KPRANGE1, PA is equal to VA with bit30~31 cleared. For example, the uncached direct-mapped VA of 0x00001000 is 0x80001000, and the cached direct-mapped VA of 0x00001000 is 0xA0001000.

By default, the whole virtual address space of LA64 is configured like this:

Name

Address Range

Attributes

XUVRANGE

0x0000000000000000 - 0x3FFFFFFFFFFFFFFF

Page-mapped, Cached, PLV0~3

XSPRANGE

0x4000000000000000 - 0x7FFFFFFFFFFFFFFF

Direct-mapped, Cached / Uncached, PLV0

XKPRANGE

0x8000000000000000 - 0xBFFFFFFFFFFFFFFF

Direct-mapped, Cached / Uncached, PLV0

XKVRANGE

0xC000000000000000 - 0xFFFFFFFFFFFFFFFF

Page-mapped, Cached, PLV0

User mode (PLV3) can only access XUVRANGE. For direct-mapped XSPRANGE and XKPRANGE, PA is equal to VA with bits 60~63 cleared, and the cache attribute is configured by bits 60~61 in VA: 0 is for strongly-ordered uncached, 1 is for coherent cached, and 2 is for weakly-ordered uncached.

Currently we only use XKPRANGE for direct mapping and XSPRANGE is reserved.

To put this in action: the strongly-ordered uncached direct-mapped VA (in XKPRANGE) of 0x00000000_00001000 is 0x80000000_00001000, the coherent cached direct-mapped VA (in XKPRANGE) of 0x00000000_00001000 is 0x90000000_00001000, and the weakly-ordered uncached direct-mapped VA (in XKPRANGE) of 0x00000000 _00001000 is 0xA0000000_00001000.

1.4. Relationship of Loongson and LoongArch

LoongArch is a RISC ISA which is different from any other existing ones, while Loongson is a family of processors. Loongson includes 3 series: Loongson-1 is the 32-bit processor series, Loongson-2 is the low-end 64-bit processor series, and Loongson-3 is the high-end 64-bit processor series. Old Loongson is based on MIPS, while New Loongson is based on LoongArch. Take Loongson-3 as an example: Loongson-3A1000/3B1500/3A2000/3A3000/3A4000 are MIPS-compatible, while Loongson- 3A5000 (and future revisions) are all based on LoongArch.

1.5. References

Official web site of Loongson Technology Corp. Ltd.:

Developer web site of Loongson and LoongArch (Software and Documentation):

Documentation of LoongArch ISA:

Documentation of LoongArch ELF psABI:

Linux kernel repository of Loongson and LoongArch: