diff --git a/asm/assembler_fuzz_test.go b/asm/assembler_fuzz_test.go
index 8fe74e8..a159477 100644
--- a/asm/assembler_fuzz_test.go
+++ b/asm/assembler_fuzz_test.go
@@ -37,6 +37,22 @@ func corpusSeeds(f *testing.F) {
 	}
 }
 
+// archCorpusSeeds seeds a target with every assembly file of its architecture
+// seed directory, testdata/seeds/<dir>.  The files hold the target-specific
+// corpus: the instruction families GOROOT's own assembler corpus exercises for
+// the architecture, minimalised, plus the boundary shapes the encoder's range
+// gates live on.  They are committed assembly, so `gasm fmt` and `gasm lint`
+// gate them the way they gate every other .s file.  A plain `go test` run
+// replays each one as a regression case.
+func archCorpusSeeds(f *testing.F, dir string) {
+	files, _ := filepath.Glob(filepath.Join("testdata", "seeds", dir, "*.s"))
+	for _, path := range files {
+		if b, err := os.ReadFile(path); err == nil {
+			f.Add(string(b))
+		}
+	}
+}
+
 // fuzzAssemble is the whole fuzz body, shared by every target and one line
 // apart between them: parse with macro and include expansion, assemble
 // through the target's file-level entry, and hold the invariants.  The
@@ -179,3 +195,86 @@ func FuzzAssembleARM64(f *testing.F) {
 		fuzzAssemble(t, "fuzz_arm64.s", src, AssembleFileARM64)
 	})
 }
+
+// FuzzAssembleRISCV64 hammers the same pipeline for the fixed riscv64 target.
+// The seed corpus lives in testdata/seeds/riscv64: the instruction families
+// GOROOT's riscv64 assembler corpus exercises (the immediate-range ladder of
+// the I-type arithmetic, the load/store and branch offsets, the atomics, the
+// FP conversions and the fused multiply-adds), the RVV configuration and
+// arithmetic classes with their mask forms, the RVC-compressible shapes, the
+// CSR instructions and the Zbb/Zba/Zbs bit-manipulation set, plus one file per
+// rejection shape so each diagnostic path replays on its own.
+func FuzzAssembleRISCV64(f *testing.F) {
+	corpusSeeds(f)
+	archCorpusSeeds(f, "riscv64")
+	// Minimal seeds for the shared file-level surface, spelled the riscv64
+	// way: the guard classes, the macro and include expansion, and the data
+	// section with its symbol-valued fields.
+	f.Add("TEXT ·f(SB), NOSPLIT, $0\n\tRET\n")
+	f.Add("TEXT ·f(SB), $16-8\n\tMOV x+0(FP), X10\n\tMOV X10, ret+8(FP)\n\tRET\n")
+	f.Add("TEXT ·f(SB), $256-0\n\tCALL ·helper(SB)\n\tRET\nTEXT ·helper(SB), NOSPLIT, $0\n\tRET\n")
+	f.Add("#define L(n) ADDI $n, X10, X10\nTEXT ·f(SB), NOSPLIT, $0\n\tL(7)\n\tRET\n")
+	f.Add("#include \"textflag.h\"\nTEXT ·f(SB), NOSPLIT, $0\n\tRET\n")
+	f.Add("#include \"fuzzdefs.h\"\nTEXT ·f(SB), $16-8\n\tMOV KONST, X10\n\tMOV ARG(x), X11\n\tRET\n")
+	f.Add("DATA d<>+0(SB)/8, $0xf4f8fcff\nDATA d<>+4(SB)/4, $1\nGLOBL d<>(SB), RODATA, $8\n" +
+		"TEXT ·f(SB), NOSPLIT, $0\n\tMOV d<>(SB), X10\n\tRET\n")
+	f.Add("TEXT ·f(SB), NOSPLIT, $0\n\tPCALIGN $16\n\tADD X11, X10, X10\n\tPCALIGN $2048\n\tRET\n")
+	f.Add("TEXT ·f(SB), $0\n\tPCDATA $0, $1\n\tFUNCDATA $0, ·meta(SB)\n\tRET\n")
+	// Shapes that must be rejected: each pins a diagnostic path the accepted
+	// seeds never reach.
+	f.Add("TEXT ·f(SB), $0\n\tBOGUSINSTR X10, X11\n\tRET\n")
+	f.Add("GLOBL d(SB), $-8\n")
+	f.Add("GLOBL d(SB), $0x4000001\n")
+	f.Add("DATA d+0(SB)/9, $1\nGLOBL d(SB), $8\n")
+	f.Add("TEXT ·f(SB), NOSPLIT, $0\n\tADD X11, X32\n\tRET\n")
+	f.Add("TEXT ·f(SB), NOSPLIT, $0\n\tADDI $2048, X5, X6\n\tADD X11, X40, X6\n\tRET\n")
+	f.Add("TEXT ·f(SB), NOSPLIT, $0\n\tCSRRW $0x1000, X5, X6\n\tRET\n")
+	f.Add("TEXT ·f(SB), NOSPLIT, $0\n\tSLLI $64, X5, X6\n\tRET\n")
+	f.Add("TEXT ·f(SB), NOSPLIT, $0\n\tVLE8V (X10), V32\n\tRET\n")
+	f.Add("#define A A\nA\n")
+
+	f.Fuzz(func(t *testing.T, src string) {
+		fuzzAssemble(t, "fuzz_riscv64.s", src, AssembleFileRISCV)
+	})
+}
+
+// FuzzAssembleLOONG64 hammers the same pipeline for the fixed loong64 target.
+// The seed corpus lives in testdata/seeds/loong64: the LSX and LASX register
+// banks with their immediate forms and range gates (the si5 compares, the
+// biased shifts, the VSHUF4I/VPERMI/VEXTRINS immediates), the ll/sc offset
+// ladder with its three encoding spans, the pointer loads and stores, the
+// atomics with their dbar forms, the branches, the bit-field instructions and
+// the register-class moves, plus one file per rejection shape.
+func FuzzAssembleLOONG64(f *testing.F) {
+	corpusSeeds(f)
+	archCorpusSeeds(f, "loong64")
+	// Minimal seeds for the shared file-level surface, spelled the loong64
+	// way.
+	f.Add("TEXT ·f(SB), NOSPLIT, $0\n\tRET\n")
+	f.Add("TEXT ·f(SB), $16-8\n\tMOVV x+0(FP), R4\n\tMOVV R4, ret+8(FP)\n\tRET\n")
+	f.Add("TEXT ·f(SB), $256-0\n\tCALL ·helper(SB)\n\tRET\nTEXT ·helper(SB), NOSPLIT, $0\n\tRET\n")
+	f.Add("#define L(n) ADDV $n, R4, R4\nTEXT ·f(SB), NOSPLIT, $0\n\tL(7)\n\tRET\n")
+	f.Add("#include \"textflag.h\"\nTEXT ·f(SB), NOSPLIT, $0\n\tRET\n")
+	f.Add("#include \"fuzzdefs.h\"\nTEXT ·f(SB), $16-8\n\tMOVV KONST, R4\n\tMOVV ARG(x), R5\n\tRET\n")
+	f.Add("DATA d<>+0(SB)/8, $0xf4f8fcff\nDATA d<>+4(SB)/4, $1\nGLOBL d<>(SB), RODATA, $8\n" +
+		"TEXT ·f(SB), NOSPLIT, $0\n\tMOVV d<>(SB), R4\n\tRET\n")
+	f.Add("TEXT ·f(SB), NOSPLIT, $0\n\tPCALIGN $16\n\tADDV R5, R4, R4\n\tPCALIGN $2048\n\tRET\n")
+	f.Add("TEXT ·f(SB), $0\n\tPCDATA $0, $1\n\tFUNCDATA $0, ·meta(SB)\n\tRET\n")
+	// Shapes that must be rejected: each pins a diagnostic path the accepted
+	// seeds never reach.
+	f.Add("TEXT ·f(SB), $0\n\tBOGUSINSTR R4, R5\n\tRET\n")
+	f.Add("GLOBL d(SB), $-8\n")
+	f.Add("GLOBL d(SB), $0x4000001\n")
+	f.Add("DATA d+0(SB)/9, $1\nGLOBL d(SB), $8\n")
+	f.Add("TEXT ·f(SB), NOSPLIT, $0\n\tADD R5, R32\n\tRET\n")
+	f.Add("TEXT ·f(SB), NOSPLIT, $0\n\tBEQZ V0, L1\nL1:\n\tRET\n")
+	f.Add("TEXT ·f(SB), NOSPLIT, $0\n\tVADDV V1, V2, X3\n\tRET\n")
+	f.Add("TEXT ·f(SB), NOSPLIT, $0\n\tVSEQV $32, V2, V3\n\tRET\n")
+	f.Add("TEXT ·f(SB), NOSPLIT, $0\n\tVSHUF4IV $16, V2, V1\n\tRET\n")
+	f.Add("TEXT ·f(SB), NOSPLIT, $0\n\tBSTRPICKV $64, R4, $5, R6\n\tRET\n")
+	f.Add("#define A A\nA\n")
+
+	f.Fuzz(func(t *testing.T, src string) {
+		fuzzAssemble(t, "fuzz_loong64.s", src, AssembleFileLOONG64)
+	})
+}
diff --git a/asm/testdata/seeds/loong64/llsc-offsets.s b/asm/testdata/seeds/loong64/llsc-offsets.s
new file mode 100644
index 0000000..30e2f2a
--- /dev/null
+++ b/asm/testdata/seeds/loong64/llsc-offsets.s
@@ -0,0 +1,84 @@
+// Copyright (c) 2026 Petr Balvín <opensource@petrbalvin.org> (https://petrbalvin.org)
+// SPDX-License-Identifier: BSD-3-Clause
+
+// Seed corpus for FuzzAssembleLOONG64: the ll/sc and pointer load/store
+// offset ladder, the three spans the encoder classifies by (a signed 16-bit
+// offset alone, a 32-bit one split through addu16i.d into R30, a wider one
+// materialised in full), the acquire/release pair, sc.q and the atomics with
+// their dbar forms.
+
+#include "textflag.h"
+
+TEXT ·llscNear(SB), NOSPLIT, $0
+	LL  (R12), R13
+	LL  0(R12), R13
+	LL  4(R12), R13
+	LLW 8(R12), R13
+	LLW -4(R12), R13
+	LLV 8(R14), R15
+	LLV -8(R14), R15
+	SC  R16, (R17)
+	SC  R16, 0(R17)
+	SCW R16, 4(R17)
+	SCV R18,-8(R19)
+	RET
+
+TEXT ·llscSpans(SB), NOSPLIT, $0
+	LL  1020(R12), R13
+	LL  1024(R12), R13
+	LL  -1024(R12), R13
+	LLW 32760(R12), R13
+	LLW 32764(R12), R13
+	LLW 32768(R12), R13
+	LLW -32768(R12), R13
+	LLV 65532(R14), R15
+	LLV 65536(R14), R15
+	LLV 131068(R14), R15
+	LLW 1048572(R12), R13
+	LLV 2147483644(R14), R15
+	LLV -2147483648(R14), R15
+	SC  R16, 1020(R17)
+	SCW R16, 32764(R17)
+	SCV R18, 65532(R19)
+	SCV R18, 65536(R19)
+	RET
+
+TEXT ·ptrForms(SB), NOSPLIT, $0
+	MOVWP R16, (R17)
+	MOVWP R16, 8(R17)
+	MOVVP R18, (R19)
+	MOVVP R18,-8(R19)
+	RET
+
+TEXT ·acqRel(SB), NOSPLIT, $0
+	LLACQW (R5), R4
+	LLACQV (R5), R4
+	SCRELW R4, (R6)
+	SCRELV R4, (R6)
+	SCQ    R4, R5, (R6)
+	RET
+
+TEXT ·atomics(SB), NOSPLIT, $0
+	AMSWAPB   R14, (R13), R12
+	AMSWAPH   R14, (R13), R12
+	AMSWAPW   R14, (R13), R12
+	AMSWAPV   R14, (R13), R12
+	AMCASB    R14, (R13), R12
+	AMCASDBV  R14, (R13), R12
+	AMADDW    R14, (R13), R12
+	AMADDV    R14, (R13), R12
+	AMANDW    R6, (R5), R4
+	AMORW     R0, (R5), R4
+	AMXORV    R14, (R13), R12
+	AMMAXW    R14, (R13), R12
+	AMMINV    R14, (R13), R12
+	AMMAXWU   R14, (R13), R12
+	AMMINVU   R14, (R13), R12
+	AMADDDBW  R14, (R13), R12
+	AMADDDBV  R14, (R13), R12
+	AMANDDBW  R6, (R5), R4
+	AMORDBV   R14, (R13), R12
+	AMXORDBW  R14, (R13), R12
+	AMMAXDBWU R14, (R13), R12
+	AMMINDBVU R14, (R13), R12
+	RET
diff --git a/asm/testdata/seeds/loong64/lsx-lasx.s b/asm/testdata/seeds/loong64/lsx-lasx.s
new file mode 100644
index 0000000..6a01da4
--- /dev/null
+++ b/asm/testdata/seeds/loong64/lsx-lasx.s
@@ -0,0 +1,212 @@
+// Copyright (c) 2026 Petr Balvín <opensource@petrbalvin.org> (https://petrbalvin.org)
+// SPDX-License-Identifier: BSD-3-Clause
+
+// Seed corpus for FuzzAssembleLOONG64: the LSX (V) and LASX (X) register
+// banks, the family GOROOT's loong64 assembler corpus exercises
+// (cmd/asm/internal/asm/testdata/loong64enc*.s).  Every immediate window sits
+// on an encoder gate: the si5 compares, the biased shifts, the ui8 logic
+// immediates and the element moves.
+
+#include "textflag.h"
+
+TEXT ·vecArith(SB), NOSPLIT, $0
+	VADDV       V1, V2, V3
+	VADDW       V1, V2, V3
+	VADDD       V1, V2, V3
+	VADDQ       V1, V2, V3
+	VSUBV       V1, V2, V3
+	VSSUBV      V1, V2, V3
+	VSADDVU     V1, V2, V3
+	VMULV       V1, V2, V3
+	VMUHVU      V1, V2, V3
+	VDIVB       V1, V2, V3
+	VMODH       V1, V2, V3
+	VANDV       V1, V2, V3
+	VORV        V1, V2, V3
+	VXORV       V1, V2, V3
+	VNORV       V1, V2, V3
+	VANDNV      V1, V2, V3
+	VSEQB       V1, V2, V3
+	VSLTW       V1, V2, V3
+	VSLTBU      V1, V2, V3
+	VSLLB       V1, V2, V3
+	VSRAH       V1, V2, V3
+	VROTRV      V1, V2, V3
+	VILVLW      V1, V2, V3
+	VILVHV      V1, V2, V3
+	VBITCLRW    V1, V2, V3
+	VBITSETB    V1, V2, V3
+	VBITREVW    V1, V2, V3
+	VADDWEVHB   V1, V2, V3
+	VSUBWODVW   V1, V2, V3
+	VMULWEVQV   V1, V2, V3
+	VMADDWODQVU V1, V2, V3
+	VSHUFV      V1, V2, V3
+	RET
+
+TEXT ·vecArithX(SB), NOSPLIT, $0
+	XVADDV        X1, X2, X3
+	XVADDW        X1, X2, X3
+	XVADDD        X1, X2, X3
+	XVADDQ        X1, X2, X3
+	XVSUBV        X1, X2, X3
+	XVSSUBVU      X1, X2, X3
+	XVMULV        X1, X2, X3
+	XVMUHVU       X1, X2, X3
+	XVDIVB        X1, X2, X3
+	XVANDV        X1, X2, X3
+	XVNORV        X1, X2, X3
+	XVSEQB        X1, X2, X3
+	XVSLTW        X1, X2, X3
+	XVSLLB        X1, X2, X3
+	XVSRAB        X1, X2, X3
+	XVROTRW       X1, X2, X3
+	XVILVLB       X1, X2, X3
+	XVBITCLRV     X1, X2, X3
+	XVADDWEVHB    X1, X2, X3
+	XVMULWODWHU   X1, X2, X3
+	XVMADDWEVQVUV X1, X2, X3
+	XVSHUFV       X1, X2, X3
+	RET
+
+TEXT ·vecImm(SB), NOSPLIT, $0
+	VANDB     $0, V2, V3
+	VANDB     $255, V2, V3
+	VORB      $128, V2, V3
+	VXORB     $255, V2, V3
+	VNORB     $1, V2, V3
+	VSEQB     $-16, V2, V3
+	VSEQB     $15, V2, V3
+	VSEQH     $0, V2, V3
+	VSEQW     $-1, V2, V3
+	VSEQV     $15, V2, V3
+	VSEQV     $-15, V2, V3
+	VSLTB     $-16, V2, V3
+	VSLTBU    $0, V2, V3
+	VSLTBU    $31, V2, V3
+	VADDBU    $0, V2, V3
+	VADDVU    $31, V2, V3
+	VSUBBU    $31, V2, V3
+	VSLLB     $0, V2, V3
+	VSLLB     $7, V2, V3
+	VSLLH     $15, V2, V3
+	VSLLW     $31, V2, V3
+	VSLLV     $63, V2, V3
+	VSRLB     $7, V2, V3
+	VSRAB     $7, V2, V3
+	VSRAH     $15, V2, V3
+	VSRAW     $31, V2, V3
+	VSRAV     $63, V2, V3
+	VROTRB    $7, V2, V3
+	VROTRH    $15, V2, V3
+	VROTRW    $31, V2, V3
+	VROTRV    $63, V2, V3
+	VBITCLRB  $7, V2, V3
+	VBITSETH  $15, V2, V3
+	VBITREVW  $31, V2, V3
+	VBITREVV  $63, V2, V3
+	XVANDB    $0, X2, X3
+	XVANDB    $255, X2, X3
+	XVSEQB    $-16, X2, X3
+	XVSEQB    $15, X2, X3
+	XVSEQV    $15, X2, X3
+	XVADDVU   $31, X2, X3
+	XVSLLB    $0, X2, X3
+	XVSLLB    $7, X2, X3
+	XVSLLV    $63, X2, X3
+	XVSRAB    $7, X2, X3
+	XVROTRV   $63, X2, X3
+	XVBITCLRB $7, X2, X3
+	XVBITSETV $63, X2, X3
+	RET
+
+TEXT ·vecUnary(SB), NOSPLIT, $0
+	VPCNTB     V1, V2
+	VPCNTH     V1, V2
+	VPCNTW     V1, V2
+	VPCNTV     V1, V2
+	VNEGB      V1, V2
+	VNEGW      V1, V2
+	VNEGV      V1, V2
+	VFCLASSF   V1, V2
+	VFCLASSD   V1, V2
+	VFSQRTF    V1, V2
+	VFSQRTD    V1, V2
+	VFRECIPD   V1, V2
+	VFRSQRTF   V1, V2
+	VFRINTF    V1, V2
+	VFRINTRMD  V1, V2
+	VFRINTRNEF V1, V2
+	VFRINTRZD  V1, V2
+	VFRINTRPF  V1, V2
+	XVPCNTB    X1, X2
+	XVPCNTV    X1, X2
+	XVNEGH     X1, X2
+	XVFSQRTD   X1, X2
+	XVFRECIPF  X1, X2
+	XVFRINTRZD X1, X2
+	RET
+
+TEXT ·vecCond(SB), NOSPLIT, $0
+	VSETNEV     V1, FCC0
+	VSETEQV     V1, FCC1
+	VSETANYEQB  V1, FCC2
+	VSETANYEQH  V2, FCC3
+	VSETANYEQW  V1, FCC4
+	VSETANYEQV  V2, FCC5
+	VSETALLNEB  V1, FCC6
+	VSETALLNEH  V2, FCC7
+	VSETALLNEW  V1, FCC0
+	VSETALLNEV  V0, FCC1
+	XVSETNEV    X1, FCC0
+	XVSETEQV    X2, FCC1
+	XVSETANYEQV X1, FCC2
+	XVSETALLNEB X2, FCC3
+	RET
+
+TEXT ·vecShuffle4(SB), NOSPLIT, $0
+	VSHUFB  V0, V1, V2, V3
+	XVSHUFB X0, X1, X2, X3
+	RET
+
+// The VMOVQ/XVMOVQ move family: the load, the indexed load, the store, the
+// replicated load, the whole-register move, the GPR broadcast, the element
+// extract, the element insert and the element broadcast.
+TEXT ·vecMove(SB), NOSPLIT, $0
+	VMOVQ  8(R4), V2
+	VMOVQ  (R4)(R5), V2
+	VMOVQ  V2, 8(R4)
+	VMOVQ  V2, (R4)(R5)
+	VMOVQ  2(R4), V1.H8
+	VMOVQ  -6(R4), V1.H8
+	VMOVQ  -12(R4), V2.W4
+	VMOVQ  V1, V2
+	VMOVQ  R4, V1.B16
+	VMOVQ  R4, V1.H8
+	VMOVQ  R4, V1.W4
+	VMOVQ  R4, V1.V2
+	VMOVQ  V1.B[3], R5
+	VMOVQ  V1.H[7], R5
+	VMOVQ  V1.W[3], R5
+	VMOVQ  V1.V[1], R5
+	VMOVQ  R5, V1.B[3]
+	VMOVQ  R5, V1.H[7]
+	VMOVQ  R5, V1.W[3]
+	VMOVQ  R5, V1.V[1]
+	VMOVQ  V1.B[3], V9.B16
+	VMOVQ  V2.H[2], V8.H8
+	VMOVQ  V3.W[1], V7.W4
+	VMOVQ  V4.V[0], V6.V2
+	XVMOVQ X1, X2
+	XVMOVQ X0, X31.B32
+	XVMOVQ X1, X30.H16
+	XVMOVQ X2, X29.W8
+	XVMOVQ X3, X28.V4
+	XVMOVQ X3, X27.Q2
+	XVMOVQ X0, X31.W[7]
+	XVMOVQ X1, X29.W[0]
+	XVMOVQ X3, X28.V[3]
+	XVMOVQ X4, X27.V[0]
+	XVMOVQ X31.W[7], X0
+	XVMOVQ X28.V[3], X8
+	RET
diff --git a/asm/testdata/seeds/loong64/misc-branches.s b/asm/testdata/seeds/loong64/misc-branches.s
new file mode 100644
index 0000000..4500f0f
--- /dev/null
+++ b/asm/testdata/seeds/loong64/misc-branches.s
@@ -0,0 +1,121 @@
+// Copyright (c) 2026 Petr Balvín <opensource@petrbalvin.org> (https://petrbalvin.org)
+// SPDX-License-Identifier: BSD-3-Clause
+
+// Seed corpus for FuzzAssembleLOONG64: the branches over their offset range,
+// the jumps, the bit-field pair at the msb/lsb boundaries, the ALSL shift
+// window, RDTIME, the system instructions, the constant materialisation
+// chain and the register-class moves.
+
+#include "textflag.h"
+
+TEXT ·branches(SB), NOSPLIT, $0
+	BEQ  R4, R5, b1
+	BNE  R4, R5, b2
+	BLT  R4, R5, b1
+	BGE  R4, R5, b2
+	BLTU R4, R5, b1
+	BGEU R4, R5, b2
+	BEQZ R6, b1
+	BNEZ R7, b2
+	BLTZ R8, b1
+	BGEZ R9, b2
+	BLEZ R10, b1
+	BGTZ R11, b2
+	BFPT b1
+	BFPF b2
+
+b1:
+	MOVV R1, R2
+
+b2:
+	MOVV R2, R1
+	BEQ  R4, R5, b1
+	RET
+
+TEXT ·jumps(SB), NOSPLIT, $0
+	JMP  j1
+	JAL  (R30)
+	JIRL R0, R4, 8
+	JIRL R1, R5, 0
+	CALL runtime·morestack_noctxt(SB)
+
+j1:
+	JMP  -3(PC)
+	RET
+
+TEXT ·bitfield(SB), NOSPLIT, $0
+	BSTRINSW  $3, R4, $0, R5
+	BSTRINSV  $7, R4, $0, R5
+	BSTRINSV  $31, R6, $0, R7
+	BSTRINSV  $63, R6, $32, R7
+	BSTRPICKW $3, R4, $0, R5
+	BSTRPICKV $6, R7, $0, R8
+	BSTRPICKV $63, R6, $32, R7
+	ALSLW     $1, R4, R5, R6
+	ALSLW     $4, R7, R8, R9
+	ALSLWU    $3, R4, R5, R6
+	ALSLV     $2, R8, R9, R10
+	ALSLV     $3, R8, R9, R10
+	RET
+
+TEXT ·system(SB), NOSPLIT, $0
+	SYSCALL
+	DBAR
+	DBAR     $0
+	BREAK    $0
+	BREAK    $32767
+	PRELD    0(R4), $0
+	PRELD    2047(R4), $31
+	RDTIMELW R7, R8
+	RDTIMEHW R7, R8
+	RDTIMED  R20, R21
+	RET
+
+TEXT ·consts(SB), NOSPLIT, $0
+	LU12IW    R4, $0x12345
+	LU12IW    R5, $-524288
+	LU12IW    R6, $1048575
+	LU32ID    R7, $-524288
+	LU52ID    $-4096, R8, R9
+	LU52ID    $4095, R8, R9
+	PCALAU12I R10, $0x12345
+	PCADDU12I R11, $-524288
+	ADDV16    $65536, R11, R12
+	ADDV16    $-65536, R11, R12
+	MOVV      $0x123456789ABCDEF0, R13
+	MOVV      $-1, R14
+	RET
+
+TEXT ·regClasses(SB), NOSPLIT, $0
+	MOVV R4, F5
+	MOVV F6, R7
+	MOVW R8, F9
+	MOVW F10, R11
+	MOVV R12, FCC0
+	MOVV FCC1, R13
+	MOVV R14, FCSR0
+	MOVV FCSR3, R15
+	FSEL FCC0, F3, F4, F3
+	FSEL FCC1, F1, F2
+	RET
+
+TEXT ·memOffsets(SB), NOSPLIT, $0
+	MOVB  2047(R4), R5
+	MOVB  -2048(R4), R5
+	MOVH  2047(R4), R5
+	MOVW  2047(R4), R5
+	MOVV  2047(R4), R5
+	MOVW  2048(R4), R5
+	MOVW  -2048(R4), R5
+	MOVW  4095(R4), R5
+	MOVW  -4096(R4), R5
+	MOVW  65536(R4), R5
+	MOVW  $4(R4), R5
+	MOVV  $4(R4), R5
+	MOVBU -1(R4), R5
+	MOVHU 2(R4), R5
+	MOVWU -3(R4), R5
+	MOVF  8(R4), F5
+	MOVD  -8(R4), F5
+	MOVV  (R4)(R5), R6
+	RET
diff --git a/asm/testdata/seeds/loong64/vshuf4iv.s b/asm/testdata/seeds/loong64/vshuf4iv.s
new file mode 100644
index 0000000..9f3fdda
--- /dev/null
+++ b/asm/testdata/seeds/loong64/vshuf4iv.s
@@ -0,0 +1,61 @@
+// Copyright (c) 2026 Petr Balvín <opensource@petrbalvin.org> (https://petrbalvin.org)
+// SPDX-License-Identifier: BSD-3-Clause
+
+// Seed corpus for FuzzAssembleLOONG64: the shuffle/permute immediate family.
+// VSHUF4I.B/H/W take a ui8 packing two position nibbles; VSHUF4I.V is
+// range-gated to 0..15 although its field is ui8, the same gate the
+// toolchain keeps.  VPERMI.W and the VEXTRINS pack an element index and an
+// insert/extract order in one ui8.
+
+#include "textflag.h"
+
+TEXT ·shuf4(SB), NOSPLIT, $0
+	VSHUF4IB $0, V2, V1
+	VSHUF4IB $16, V2, V1
+	VSHUF4IB $255, V2, V1
+	VSHUF4IH $0, V2, V1
+	VSHUF4IH $128, V2, V1
+	VSHUF4IH $255, V2, V1
+	VSHUF4IW $0, V2, V1
+	VSHUF4IW $96, V2, V1
+	VSHUF4IW $255, V2, V1
+	VSHUF4IV $0, V2, V1
+	VSHUF4IV $8, V2, V1
+	VSHUF4IV $15, V2, V1
+	RET
+
+TEXT ·shuf4X(SB), NOSPLIT, $0
+	XVSHUF4IB $0, X2, X1
+	XVSHUF4IB $255, X2, X1
+	XVSHUF4IH $255, X2, X1
+	XVSHUF4IW $255, X2, X1
+	XVSHUF4IV $0, X2, X1
+	XVSHUF4IV $15, X2, X1
+	RET
+
+TEXT ·permi(SB), NOSPLIT, $0
+	VPERMIW  $0, V2, V1
+	VPERMIW  $255, V2, V1
+	XVPERMIW $0, X2, X1
+	XVPERMIW $255, X2, X1
+	XVPERMIV $0, X2, X1
+	XVPERMIV $255, X2, X1
+	XVPERMIQ $0, X2, X1
+	XVPERMIQ $255, X2, X1
+	RET
+
+TEXT ·extrins(SB), NOSPLIT, $0
+	VEXTRINSB  $0, V2, V1
+	VEXTRINSB  $255, V2, V1
+	VEXTRINSH  $0, V2, V1
+	VEXTRINSH  $255, V2, V1
+	VEXTRINSW  $0, V2, V1
+	VEXTRINSW  $255, V2, V1
+	VEXTRINSV  $0, V2, V1
+	VEXTRINSV  $255, V2, V1
+	XVEXTRINSB $0, X2, X1
+	XVEXTRINSB $255, X2, X1
+	XVEXTRINSH $255, X2, X1
+	XVEXTRINSW $255, X2, X1
+	XVEXTRINSV $255, X2, X1
+	RET
diff --git a/asm/testdata/seeds/riscv64/csr.s b/asm/testdata/seeds/riscv64/csr.s
new file mode 100644
index 0000000..e75d65a
--- /dev/null
+++ b/asm/testdata/seeds/riscv64/csr.s
@@ -0,0 +1,35 @@
+// Copyright (c) 2026 Petr Balvín <opensource@petrbalvin.org> (https://petrbalvin.org)
+// SPDX-License-Identifier: BSD-3-Clause
+
+// Seed corpus for FuzzAssembleRISCV64: the CSR instructions over the whole
+// 12-bit address range and the 5-bit immediate window of the CSRRWI family.
+
+#include "textflag.h"
+
+TEXT ·csrRegs(SB), NOSPLIT, $0
+	CSRRW $0x300, X5, X6
+	CSRRS $0x304, X7, X8
+	CSRRC $0x305, X9, X10
+	CSRRW $0, X5, X6
+	CSRRS $0xFFF, X7, X8
+	CSRRC $0x7FF, X9, X10
+	CSRRW $0x341, X0, X6
+	CSRRS $0x342, X0, X6
+	RET
+
+TEXT ·csrImm(SB), NOSPLIT, $0
+	CSRRWI $0x300, $0, X6
+	CSRRSI $0x304, $31, X8
+	CSRRCI $0x305, $1, X10
+	CSRRWI $0xFFF, $15, X6
+	CSRRSI $0, $31, X8
+	CSRRCI $0x7FF, $16, X10
+	RET
+
+TEXT ·csrNamed(SB), NOSPLIT, $0
+	CSRRS  $0xC00, X0, X14
+	CSRRS  $0xC01, X0, X15
+	CSRRS  $0xC02, X0, X16
+	CSRRW  $0x105, X5, X6
+	CSRRSI $0x106, $1, X7
+	RET
diff --git a/asm/testdata/seeds/riscv64/imm-ranges.s b/asm/testdata/seeds/riscv64/imm-ranges.s
new file mode 100644
index 0000000..cffa8ad
--- /dev/null
+++ b/asm/testdata/seeds/riscv64/imm-ranges.s
@@ -0,0 +1,106 @@
+// Copyright (c) 2026 Petr Balvín <opensource@petrbalvin.org> (https://petrbalvin.org)
+// SPDX-License-Identifier: BSD-3-Clause
+
+// Seed corpus for FuzzAssembleRISCV64: the immediate-range ladder of the
+// I-type arithmetic and the U-type pair, the family GOROOT's riscv64
+// assembler corpus walks (cmd/asm/internal/asm/testdata/riscv64.s).  Every
+// boundary sits on an encoder gate: the 12-bit signed window, the
+// one-instruction and two-instruction decompositions, the shamt widths and
+// the wide MOV immediate that goes through the literal pool.
+
+#include "textflag.h"
+
+TEXT ·immIType(SB), NOSPLIT, $0
+	ADDI  $2047, X5, X6
+	ADDI  $-2048, X5, X6
+	ADDI  $2048, X5, X6
+	ADDI  $-2049, X5, X6
+	ADDI  $4094, X5, X6
+	ADDI  $-4096, X5, X6
+	ADDI  $4095, X5, X6
+	ADDI  $-4097, X5, X6
+	ADDI  $0, X5, X6
+	SLTI  $2047, X5, X7
+	SLTI  $-2048, X5, X7
+	SLTIU $2047, X5, X7
+	SLTIU $-2048, X5, X7
+	ANDI  $-2048, X5, X6
+	ANDI  $2047, X5, X6
+	ORI   $-1, X5, X6
+	XORI  $2047, X5, X6
+	XORI  $-2048, X5, X6
+	RET
+
+TEXT ·immAlias(SB), NOSPLIT, $0
+	ADD  $2047, X5, X6
+	ADD  $2048, X5, X6
+	SUB  $1, X5, X6
+	SUB  $2049, X5, X6
+	AND  $-1, X5, X6
+	OR   $2047, X5, X6
+	XOR  $-2048, X5, X6
+	SLT  $55, X5, X7
+	SLTU $55, X5, X7
+	ADDW $2047, X5, X6
+	RET
+
+TEXT ·immUType(SB), NOSPLIT, $0
+	LUI   $0x7FFFF, X6
+	LUI   $-0x80000, X6
+	LUI   $1, X6
+	AUIPC $0x7FFFF, X7
+	AUIPC $-0x80000, X7
+	RET
+
+TEXT ·immShift(SB), NOSPLIT, $0
+	SLLI   $0, X5, X6
+	SLLI   $31, X5, X6
+	SLLI   $63, X5, X6
+	SRLI   $31, X5, X6
+	SRLI   $63, X5, X6
+	SRAI   $1, X5, X6
+	SRAI   $63, X5, X6
+	SLLIW  $31, X5, X6
+	SRLIW  $31, X5, X6
+	SRAIW  $31, X5, X6
+	SLLIUW $31, X5, X6
+	RET
+
+TEXT ·immWide(SB), NOSPLIT, $0
+	MOV $0, X10
+	MOV $2047, X10
+	MOV $2048, X10
+	MOV $-2048, X10
+	MOV $4095, X10
+	MOV $-4097, X10
+	MOV $0x12345678, X10
+	MOV $0x123456789, X10
+	MOV $0x123456789ABCDEF0, X10
+	MOV $-1, X11
+	MOV $-0x8000000000000000, X11
+	MOV $0x7FFFFFFFFFFFFFFF, X12
+	RET
+
+TEXT ·memOffsets(SB), NOSPLIT, $0
+	LD  0(X10), X11
+	LD  2047(X10), X11
+	LD  -2048(X10), X11
+	LD  2048(X10), X11
+	LD  4095(X10), X11
+	LD  -4096(X10), X11
+	LW  0(X10), X12
+	LBU -1(X10), X12
+	LHU 2(X10), X12
+	LWU -3(X10), X12
+	SD  X11, 0(X10)
+	SD  X11, 2047(X10)
+	SD  X11,-2048(X10)
+	SD  X11, 4095(X10)
+	SW  X12,-4096(X10)
+	SH  X12, 6(X10)
+	SB  X12, 7(X10)
+	FLW 8(X10), F8
+	FLD -8(X10), F8
+	FSW F8, 16(X10)
+	FSD F8,-16(X10)
+	RET
diff --git a/asm/testdata/seeds/riscv64/rvc.s b/asm/testdata/seeds/riscv64/rvc.s
new file mode 100644
index 0000000..d612171
--- /dev/null
+++ b/asm/testdata/seeds/riscv64/rvc.s
@@ -0,0 +1,95 @@
+// Copyright (c) 2026 Petr Balvín <opensource@petrbalvin.org> (https://petrbalvin.org)
+// SPDX-License-Identifier: BSD-3-Clause
+
+// Seed corpus for FuzzAssembleRISCV64: the RVC-compressible shapes.  Every
+// instruction here has a compressed 16-bit encoding the compression pass may
+// pick: the two-operand ALU forms over x8-x15, the stack-pointer loads and
+// stores at the scaled offset boundaries, C.ADDI16SP and C.ADDI4SPN ranges,
+// the compressed branches and the uncompressible spellings beside them.
+
+#include "textflag.h"
+
+TEXT ·primeALU(SB), NOSPLIT, $0
+	ADD  X9, X8, X8
+	ADD  X15, X8, X8
+	ADDI $1, X8, X8
+	ADDI $31, X15, X15
+	ADDI $-32, X8, X8
+	AND  X9, X8, X8
+	ANDI $3, X8, X8
+	OR   X10, X9, X9
+	XOR  X11, X10, X10
+	SUB  X13, X12, X12
+	SLLI $2, X8, X8
+	SRLI $5, X9, X9
+	SRAI $7, X10, X10
+	MOV  X9, X8
+	MOV  X1, X8
+	MOV  X8, X1
+	RET
+
+TEXT ·stackOffsets(SB), NOSPLIT, $0
+	LW  0(X2), X8
+	LW  4(X2), X8
+	LW  124(X2), X9
+	LW  2044(X2), X10
+	LD  0(X2), X8
+	LD  8(X2), X8
+	LD  504(X2), X9
+	SW  X8, 0(X2)
+	SW  X8, 124(X2)
+	SW  X8, 2044(X2)
+	SD  X8, 0(X2)
+	SD  X8, 504(X2)
+	FLW 0(X2), F8
+	FLD 504(X2), F8
+	FSW F8, 0(X2)
+	FSD F8, 504(X2)
+	RET
+
+TEXT ·frameAdj(SB), NOSPLIT, $0
+	ADDI $16, X2, X2
+	ADDI $-16, X2, X2
+	ADDI $496, X2, X2
+	ADDI $-512, X2, X2
+	ADDI $1024, X2, X2
+	ADDI $2048, X2, X2
+	ADDI $1, X2, X2
+	RET
+
+TEXT ·primeMem(SB), NOSPLIT, $0
+	LW  0(X8), X9
+	LW  4(X8), X10
+	LW  124(X8), X11
+	LD  0(X8), X9
+	LD  8(X8), X10
+	LD  120(X8), X11
+	SW  X9, 0(X8)
+	SW  X9, 124(X8)
+	SD  X9, 0(X8)
+	SD  X9, 120(X8)
+	RET
+
+TEXT ·cBranches(SB), NOSPLIT, $0
+	BEQ  X8, X0, cb1
+	BNE  X9, X0, cb2
+	BLT  X10, X0, cb1
+	BGE  X11, X0, cb2
+	BLTU X12, X0, cb1
+	BGEU X13, X0, cb2
+
+cb1:
+	MOV  X8, X9
+
+cb2:
+	MOV  X9, X8
+	RET
+
+TEXT ·uncompressible(SB), NOSPLIT, $0
+	ADD  X17, X16, X16
+	ADDI $2048, X8, X8
+	LW   2048(X2), X10
+	LD   512(X2), X10
+	SW   X8, 2048(X2)
+	ADDI $2049, X2, X2
+	RET
diff --git a/asm/testdata/seeds/riscv64/rvv.s b/asm/testdata/seeds/riscv64/rvv.s
new file mode 100644
index 0000000..f78d530
--- /dev/null
+++ b/asm/testdata/seeds/riscv64/rvv.s
@@ -0,0 +1,233 @@
+// Copyright (c) 2026 Petr Balvín <opensource@petrbalvin.org> (https://petrbalvin.org)
+// SPDX-License-Identifier: BSD-3-Clause
+
+// Seed corpus for FuzzAssembleRISCV64: the RVV surface.  The configuration
+// setters, the load/store families with their segment and fault-only-first
+// variants, the arithmetic operand classes (VV/VX/VI, the masks, the
+// merge and carry forms, the reductions and the whole-register moves).
+
+#include "textflag.h"
+
+TEXT ·config(SB), NOSPLIT, $0
+	VSETVLI  X5, E8, M8, TA, MA, X6
+	VSETVLI  X5, E8, M1, TU, MU, X6
+	VSETVLI  $15, E32, M1, TA, MA, X12
+	VSETVLI  $31, E64, M2, TU, MU, X12
+	VSETIVLI $0, E16, M4, TA, MA, X0
+	VSETIVLI $4, E32, M1, TA, MA, X0
+	VSETIVLI $31, E8, M8, TU, MU, X15
+	RET
+
+TEXT ·loads(SB), NOSPLIT, $0
+	VLE8V     (X10), V8
+	VLE16V    (X10), V8
+	VLE32V    (X10), V8
+	VLE64V    (X11), V16
+	VSE8V     V8, (X10)
+	VSE16V    V8, (X10)
+	VSE32V    V15, (X11)
+	VSE64V    V24, (X10)
+	VLSE8V    (X14), X9, V0
+	VLSE32V   (X14), X5, V4
+	VSSE64V   V4, X7, (X14)
+	VLUXEI8V  (X10), V20, V4
+	VLOXEI32V (X10), V20, V4
+	VSUXEI64V V4, V20, (X10)
+	VSOXEI16V V4, V20, (X10)
+	VLE8FFV   (X10), V8
+	RET
+
+TEXT ·segments(SB), NOSPLIT, $0
+	VLSEG2E8V     (X14), V0
+	VLSEG4E32V    (X14), V0
+	VLSEG8E64V    (X14), V4
+	VSSEG2E32V    V0, (X14)
+	VSSEG8E8V     V4, (X14)
+	VLSSEG4E32V   (X14), X0, V0
+	VLSSEG8E32V   (X10), X0, V4
+	VLUXSEG2EI32V (X10), V20, V4
+	VLOXSEG4EI64V (X10), V20, V4
+	VSUXSEG2EI8V  V4, V20, (X10)
+	VLSEG4E8FFV   (X14), V0
+	RET
+
+TEXT ·wholeReg(SB), NOSPLIT, $0
+	VL1RV    (X10), V8
+	VL2RV    (X10), V16
+	VL4RV    (X10), V0
+	VL8RV    (X10), V0
+	VS1RV    V8, (X10)
+	VS2RV    V16, (X10)
+	VS4RV    V0, (X10)
+	VS8RV    V0, (X10)
+	VL1RE8V  (X10), V8
+	VL2RE16V (X10), V16
+	VL4RE32V (X10), V0
+	VL8RE64V (X10), V0
+	VL2RE64V (X10), V16
+	VLMV     (X10), V0
+	VSMV     V0, (X10)
+	RET
+
+TEXT ·arith(SB), NOSPLIT, $0
+	VADDVV  V1, V2, V3
+	VADDVV  V1, V2, V0, V3
+	VADDVX  X10, V2, V0, V3
+	VADDVI  $15, V2, V0, V3
+	VADDVI  $-16, V2, V0, V3
+	VADDVI  $0, V2, V3
+	VSUBVV  V1, V2, V0, V3
+	VSUBVX  X10, V2, V3
+	VRSUBVX X10, V2, V0, V3
+	VRSUBVI $15, V2, V0, V3
+	VANDVI  $-1, V2, V3
+	VORVI   $15, V2, V3
+	VORVI   $-16, V2, V3
+	VXORVI  $0, V2, V3
+	VNEGV   V2, V0, V3
+	VNOTV   V2, V3
+	VIDV    V12
+	VFMVVF  F10, V14
+	VMVVI   $15, V14
+	VMVVI   $-16, V14
+	VMVVV   V1, V4
+	VMVVX   X10, V4
+	VMVXS   V4, X10
+	VFMVSF  F10, X11
+	RET
+
+TEXT ·shiftsSliding(SB), NOSPLIT, $0
+	VSLLVI         $0, V28, V30
+	VSLLVI         $31, V28, V30
+	VSRLVI         $25, V29, V29
+	VSRAVI         $31, V4, V5
+	VNSRLWI        $31, V6, V7
+	VNSRAWI        $0, V6, V7
+	VSLLVV         V1, V2, V3
+	VSRLVX         X10, V2, V3
+	VSRAVV         V1, V2, V3
+	VRORVI         $7, V4, V5
+	VRORVI         $31, V4, V5
+	VRORVV         V1, V2, V3
+	VROLVX         X10, V2, V3
+	VSLIDEUPVI     $31, V4, V5
+	VSLIDEUPVX     X10, V4, V5
+	VSLIDEDOWNVI   $0, V4, V5
+	VSLIDEDOWNVX   X10, V4, V5
+	VFSLIDE1UPVF   F10, V4, V5
+	VFSLIDE1DOWNVF F10, V4, V5
+	VRGATHERVI     $31, V2, V3
+	VRGATHERVV     V1, V2, V3
+	VRGATHERVX     X10, V2, V3
+	RET
+
+TEXT ·compareMask(SB), NOSPLIT, $0
+	VMSEQVV  V1, V2, V0
+	VMSEQVX  X10, V2, V0
+	VMSEQVI  $0, V2, V0
+	VMSNEVI  $-16, V2, V0
+	VMSLTVI  $15, V2, V0
+	VMSLTUVI $15, V2, V0
+	VMSLEVI  $-1, V2, V0
+	VMSGEVI  $15, V2, V0
+	VMSGEUVI $0, V2, V0
+	VMSGTVV  V1, V2, V0
+	VMSLEVV  V1, V2, V0
+	VMFEQVF  F10, V2, V0
+	VMFLTVF  F10, V2, V0
+	VMFGEVF  F10, V2, V0
+	VMANDMM  V1, V2, V3
+	VMNANDMM V1, V2, V3
+	VMANDNMM V1, V2, V3
+	VMORMM   V1, V2, V3
+	VMXORMM  V1, V2, V3
+	VMXNORMM V1, V2, V3
+	VMNORMM  V1, V2, V3
+	VMORNMM  V1, V2, V3
+	VMCLRM   V1
+	VMSETM   V1
+	VMNOTM   V1, V2
+	VCPOPM   V0, X6
+	VFIRSTM  V0, X7
+	VIOTAM   V0, X5
+	VMSBFM   V0, V1
+	VMSIFM   V0, V1
+	VMSOFM   V0, V1
+	RET
+
+TEXT ·carryMerge(SB), NOSPLIT, $0
+	VADCVVM    V1, V2, V0, V3
+	VADCVXM    X10, V2, V0, V3
+	VADCVIM    $15, V2, V0, V3
+	VMADCVV    V1, V2, V3
+	VMADCVX    X10, V2, V3
+	VMADCVI    $-16, V2, V3
+	VSBCVVM    V1, V2, V0, V3
+	VSBCVXM    X10, V2, V0, V3
+	VSBCVVM    V1, V2, V0, V3
+	VSBCVXM    X10, V2, V0, V3
+	VMADCVV    V1, V2, V3
+	VMADCVX    X10, V2, V3
+	VMADCVI    $0, V2, V3
+	VMADCVVM   V1, V2, V0, V3
+	VMERGEVVM  V1, V2, V0, V3
+	VMERGEVXM  X10, V2, V0, V3
+	VMERGEVIM  $15, V2, V0, V3
+	VFMERGEVFM F10, V2, V0, V3
+	RET
+
+TEXT ·reduceWiden(SB), NOSPLIT, $0
+	VREDSUMVS    V1, V2, V3
+	VREDMAXUVS   V1, V2, V3
+	VREDMINVS    V1, V2, V3
+	VREDANDVS    V1, V2, V3
+	VREDORVS     V1, V2, V3
+	VREDXORVS    V1, V2, V3
+	VWREDSUMUVS  V1, V2, V3
+	VWREDSUMVS   V1, V2, V3
+	VFWREDOSUMVS V1, V2, V3
+	VFWREDUSUMVS V1, V2, V3
+	VWADDUVV     V1, V2, V3
+	VWADDWV      V1, V2, V3
+	VWSUBUVV     V1, V2, V3
+	VWSUBWV      V1, V2, V3
+	VWADDUWX     X10, V2, V3
+	VWADDWX      X10, V2, V3
+	VMULVV       V1, V2, V3
+	VMULHUVV     V1, V2, V3
+	VMULHSUVV    V1, V2, V3
+	VDIVUVV      V1, V2, V3
+	VREMVV       V1, V2, V3
+	VSMULVV      V1, V2, V3
+	VCLMULVV     V1, V2, V3
+	VCLMULHVX    X10, V2, V3
+	RET
+
+TEXT ·fpVec(SB), NOSPLIT, $0
+	VFADDVV     V1, V2, V3
+	VFADDVF     F10, V2, V3
+	VFSUBVF     F10, V2, V3
+	VFMULVV     V1, V2, V3
+	VFDIVVF     F10, V2, V3
+	VFRSUBVF    F10, V2, V3
+	VFMINVV     V1, V2, V3
+	VFMAXVF     F10, V2, V3
+	VFMACCVV    V1, V2, V3
+	VFMACCVF    F10, V2, V3
+	VFMSACVV    V1, V2, V3
+	VFNMSACVF   F10, V2, V3
+	VFMADDVV    V1, V2, V3
+	VFNMADDVF   F10, V2, V3
+	VFWMACCVV   V1, V2, V3
+	VFWMSACVF   F10, V2, V3
+	VFSQRTV     V4, V5
+	VFCLASSV    V4, V5
+	VFREC7V     V4, V5
+	VFRSQRT7V   V4, V5
+	VFCVTXFV    V4, V5
+	VFCVTRTZXFV V4, V5
+	VFNCVTFFW   V4, V5
+	VFWCVTFFV   V4, V5
+	VSEXTVF2    V4, V5
+	VZEXTVF8    V4, V5
+	RET
diff --git a/asm/testdata/seeds/riscv64/system-fp.s b/asm/testdata/seeds/riscv64/system-fp.s
new file mode 100644
index 0000000..6628351
--- /dev/null
+++ b/asm/testdata/seeds/riscv64/system-fp.s
@@ -0,0 +1,177 @@
+// Copyright (c) 2026 Petr Balvín <opensource@petrbalvin.org> (https://petrbalvin.org)
+// SPDX-License-Identifier: BSD-3-Clause
+
+// Seed corpus for FuzzAssembleRISCV64: branches over their offset range, the
+// jumps and calls, the system instructions, the atomics with the LR/SC pair,
+// the FP arithmetic, compare, conversion and fused multiply-add families in
+// every width including quad.
+
+#include "textflag.h"
+
+TEXT ·branches(SB), NOSPLIT, $0
+	BEQ  X5, X6, b1
+	BNE  X5, X6, b2
+	BLT  X5, X6, b1
+	BGE  X5, X6, b2
+	BLTU X5, X6, b1
+	BGEU X5, X6, b2
+	BGT  X6, X5, b1
+	BLE  X6, X5, b2
+	BGTU X6, X5, b1
+	BLEU X6, X5, b2
+
+b1:
+	ADDI $1, X5, X5
+
+b2:
+	ADDI $-1, X6, X6
+	BEQ  X5, X6, b1
+	RET
+
+TEXT ·farBranch(SB), NOSPLIT, $0
+	BEQ   X5, X6, far
+	SLTI  $2047, X5, X6
+	BEQ   X5, X6, far
+
+far:
+	SLTIU $2047, X5, X6
+	BGE   X5, X6, far
+	RET
+
+TEXT ·jumps(SB), NOSPLIT, $0
+	JAL  X1, j1
+
+j1:
+	JALR X0, (X1)
+	JALR X1, 0(X5)
+	JMP  j2
+
+j2:
+	JMP  (X5)
+	CALL (X5)
+	JMP  -3(PC)
+	CALL runtime·morestack_noctxt(SB)
+	RET
+
+TEXT ·system(SB), NOSPLIT, $0
+	FENCE
+	FENCE.TSO
+	PAUSE
+	ECALL
+	EBREAK
+	RET
+
+TEXT ·atomics(SB), NOSPLIT, $0
+	AMOSWAPW X5, (X6), X7
+	AMOSWAPD X5, (X6), X7
+	AMOADDW  X5, (X6), X7
+	AMOADDD  X5, (X6), X7
+	AMOANDW  X5, (X6), X7
+	AMOORW   X5, (X6), X7
+	AMOXORW  X5, (X6), X7
+	AMOMINW  X5, (X6), X7
+	AMOMAXW  X5, (X6), X7
+	AMOMINUW X5, (X6), X7
+	AMOMAXUW X5, (X6), X7
+	AMOMIND  X5, (X6), X7
+	AMOMAXD  X5, (X6), X7
+	AMOMINUD X5, (X6), X7
+	AMOMAXUD X5, (X6), X7
+	LRW      (X6), X7
+	LRD      (X6), X7
+	SCW      X5, (X6), X7
+	SCD      X5, (X6), X7
+	RET
+
+TEXT ·fpArith(SB), NOSPLIT, $0
+	FADDS  F1, F2, F3
+	FSUBS  F1, F2, F3
+	FMULS  F1, F2, F3
+	FDIVS  F1, F2, F3
+	FADDD  F1, F2, F3
+	FSUBD  F1, F2, F3
+	FMULD  F1, F2, F3
+	FDIVD  F1, F2, F3
+	FADDQ  F1, F2, F3
+	FSUBQ  F1, F2, F3
+	FMULQ  F1, F2, F3
+	FDIVQ  F1, F2, F3
+	FSQRTS F1, F2
+	FSQRTD F1, F2
+	FSQRTQ F1, F2
+	FMINS  F1, F2, F3
+	FMAXS  F1, F2, F3
+	FMIND  F1, F2, F3
+	FMAXD  F1, F2, F3
+	FMINQ  F1, F2, F3
+	FMAXQ  F1, F2, F3
+	RET
+
+TEXT ·fpFma(SB), NOSPLIT, $0
+	FMADDS  F1, F2, F3, F4
+	FMSUBS  F1, F2, F3, F4
+	FNMSUBS F1, F2, F3, F4
+	FNMADDS F1, F2, F3, F4
+	FMADDD  F1, F2, F3, F4
+	FMSUBD  F1, F2, F3, F4
+	FNMSUBD F1, F2, F3, F4
+	FNMADDD F1, F2, F3, F4
+	FMADDQ  F1, F2, F3, F4
+	FMSUBQ  F1, F2, F3, F4
+	FNMSUBQ F1, F2, F3, F4
+	FNMADDQ F1, F2, F3, F4
+	RET
+
+TEXT ·fpCvt(SB), NOSPLIT, $0
+	FCVTWS  F1, X5
+	FCVTWUS F1, X5
+	FCVTLS  F1, X5
+	FCVTLUS F1, X5
+	FCVTWD  F1, X5
+	FCVTWUD F1, X5
+	FCVTLD  F1, X5
+	FCVTLUD F1, X5
+	FCVTSW  X5, F1
+	FCVTSWU X5, F1
+	FCVTSL  X5, F1
+	FCVTSLU X5, F1
+	FCVTDW  X5, F1
+	FCVTDWU X5, F1
+	FCVTDL  X5, F1
+	FCVTDLU X5, F1
+	FCVTSD  F1, F2
+	FCVTDS  F1, F2
+	FCVTSQ  F1, F2
+	FCVTDQ  F1, F2
+	FCVTQS  F1, F2
+	FCVTQD  F1, F2
+	FCVTWQ  F1, X5
+	FCVTLQ  F1, X5
+	FCVTQW  X5, F1
+	FCVTQL  X5, F1
+	FCLASSS F1, X5
+	FCLASSD F1, X5
+	FCLASSQ F1, X5
+	FMVXD   F1, X5
+	FMVDX   X5, F1
+	FMVXW   F1, X5
+	FMVWX   X5, F1
+	RET
+
+TEXT ·fpSign(SB), NOSPLIT, $0
+	FSGNJS  F1, F0, F2
+	FSGNJNS F1, F0, F2
+	FSGNJXS F1, F0, F2
+	FSGNJD  F1, F0, F2
+	FSGNJND F1, F0, F2
+	FSGNJXD F1, F0, F2
+	FLTS    F1, F2, X5
+	FLES    F1, F2, X5
+	FEQS    F1, F2, X5
+	FLTD    F1, F2, X5
+	FLED    F1, F2, X5
+	FEQD    F1, F2, X5
+	FLTQ    F1, F2, X5
+	FLEQ    F1, F2, X5
+	FEQQ    F1, F2, X5
+	RET
diff --git a/asm/testdata/seeds/riscv64/zbb.s b/asm/testdata/seeds/riscv64/zbb.s
new file mode 100644
index 0000000..89e3139
--- /dev/null
+++ b/asm/testdata/seeds/riscv64/zbb.s
@@ -0,0 +1,65 @@
+// Copyright (c) 2026 Petr Balvín <opensource@petrbalvin.org> (https://petrbalvin.org)
+// SPDX-License-Identifier: BSD-3-Clause
+
+// Seed corpus for FuzzAssembleRISCV64: the Zbb/Zba/Zbs/Zicond bit-manipulation
+// and address-generation set, with every unary form's W variant and the
+// shamt-range immediate forms.
+
+#include "textflag.h"
+
+TEXT ·unary(SB), NOSPLIT, $0
+	CLZ   X5, X6
+	CLZW  X5, X6
+	CTZ   X5, X6
+	CTZW  X5, X6
+	CPOP  X5, X6
+	CPOPW X5, X6
+	SEXTB X5, X6
+	SEXTH X5, X6
+	ZEXTH X5, X6
+	ORCB  X5, X6
+	REV8  X5, X6
+	RET
+
+TEXT ·rolRor(SB), NOSPLIT, $0
+	ROL    X7, X5, X6
+	ROR    X7, X5, X6
+	RORI   $0, X5, X6
+	RORI   $63, X5, X6
+	ANDN   X7, X5, X6
+	ORN    X7, X5, X6
+	XNOR   X7, X5, X6
+	MAX    X7, X5, X6
+	MAXU   X7, X5, X6
+	MIN    X7, X5, X6
+	MINU   X7, X5, X6
+	CLMUL  X7, X5, X6
+	CLMULH X7, X5, X6
+	CLMULR X7, X5, X6
+	RET
+
+TEXT ·addrGen(SB), NOSPLIT, $0
+	SH1ADD   X7, X5, X6
+	SH2ADD   X7, X5, X6
+	SH3ADD   X7, X5, X6
+	SH1ADDUW X7, X5, X6
+	SH2ADDUW X7, X5, X6
+	SH3ADDUW X7, X5, X6
+	ADDUW    X7, X5, X6
+	RET
+
+TEXT ·bitImm(SB), NOSPLIT, $0
+	BCLR  $31, X5, X6
+	BEXT  $63, X5, X6
+	BINV  $0, X5, X6
+	BSET  $31, X5, X6
+	BCLRI $63, X5, X6
+	BEXTI $31, X5, X6
+	BINVI $63, X5, X6
+	BSETI $63, X5, X6
+	RET
+
+TEXT ·condZero(SB), NOSPLIT, $0
+	CZEROEQZ X7, X5, X6
+	CZERONEZ X7, X5, X6
+	RET