Add support for the RV32E base ISA variant

The RV32E variant reduces the general-purpose registers to 16, optimizing
the architecture for resource-constrained embedded microcontrollers.

This addition enhances compatibility with the RISC-V specification and
targets low-power, minimal-complexity systems.

Author: eleanorLYJ

Makefile

@@ -127,6 +127,10 @@ endif
 ENABLE_EXT_C ?= 1
 $(call set-feature, EXT_C)
 
+# RV32E Base Integer Instruction Set
+ENABLE_RV32E ?= 0
+$(call set-feature, RV32E)
+
 # Control and Status Register (CSR)
 ENABLE_Zicsr ?= 1
 $(call set-feature, Zicsr)

src/feature.h

@@ -27,6 +27,11 @@
 #define RV32_FEATURE_EXT_C 1
 #endif
 
+/* RV32E Base Integer Instruction Set */
+#ifndef RV32_FEATURE_RV32E
+#define RV32_FEATURE_RV32E 0
+#endif
+
 /* Control and Status Register (CSR) */
 #ifndef RV32_FEATURE_Zicsr
 #define RV32_FEATURE_Zicsr 1

src/riscv.c

@@ -769,10 +769,15 @@ void rv_reset(riscv_t *rv, riscv_word_t pc)
     rv->csr_mtvec = 0;
     rv->csr_cycle = 0;
     rv->csr_mstatus = 0;
-    rv->csr_misa |= MISA_SUPER | MISA_USER | MISA_I;
+    rv->csr_misa |= MISA_SUPER | MISA_USER;
     rv->csr_mvendorid = RV_MVENDORID;
     rv->csr_marchid = RV_MARCHID;
     rv->csr_mimpid = RV_MIMPID;
+#if !RV32_HAS(RV32E)
+    rv->csr_misa |= MISA_I;
+#else
+    rv->csr_misa |= MISA_E;
+#endif
 #if RV32_HAS(EXT_A)
     rv->csr_misa |= MISA_A;
 #endif

src/riscv.h

@@ -33,39 +33,42 @@
 extern "C" {
 #endif
 
-#define RV_REGS_LIST                                   \
-    _(zero) /* hard-wired zero, ignoring any writes */ \
-    _(ra)   /* return address */                       \
-    _(sp)   /* stack pointer */                        \
-    _(gp)   /* global pointer */                       \
-    _(tp)   /* thread pointer */                       \
-    _(t0)   /* temporary/alternate link register */    \
-    _(t1)   /* temporaries */                          \
-    _(t2)                                              \
-    _(s0) /* saved register/frame pointer */           \
-    _(s1)                                              \
-    _(a0) /* function arguments / return values */     \
-    _(a1)                                              \
-    _(a2) /* function arguments */                     \
-    _(a3)                                              \
-    _(a4)                                              \
-    _(a5)                                              \
-    _(a6)                                              \
-    _(a7)                                              \
-    _(s2) /* saved register */                         \
-    _(s3)                                              \
-    _(s4)                                              \
-    _(s5)                                              \
-    _(s6)                                              \
-    _(s7)                                              \
-    _(s8)                                              \
-    _(s9)                                              \
-    _(s10)                                             \
-    _(s11)                                             \
-    _(t3) /* temporary register */                     \
-    _(t4)                                              \
-    _(t5)                                              \
-    _(t6)
+/* clang-format off */
+#define RV_REGS_LIST                                                     \
+    _(zero) /* hard-wired zero, ignoring any writes */                   \
+    _(ra)   /* return address */                                         \
+    _(sp)   /* stack pointer */                                          \
+    _(gp)   /* global pointer */                                         \
+    _(tp)   /* thread pointer */                                         \
+    _(t0)   /* temporary/alternate link register */                      \
+    _(t1)   /* temporaries */                                            \
+    _(t2)                                                                \
+    _(s0) /* saved register/frame pointer */                             \
+    _(s1)                                                                \
+    _(a0) /* function arguments / return values */                       \
+    _(a1)                                                                \
+    _(a2) /* function arguments */                                       \
+    _(a3)                                                                \
+    _(a4)                                                                \
+    _(a5)                                                                \
+    IIF(RV32_HAS(RV32E))(,                                               \
+        _(a6)                                                            \
+        _(a7)                                                            \
+        _(s2) /* saved register */                                       \
+        _(s3)                                                            \
+        _(s4)                                                            \
+        _(s5)                                                            \
+        _(s6)                                                            \
+        _(s7)                                                            \
+        _(s8)                                                            \
+        _(s9)                                                            \
+        _(s10)                                                           \
+        _(s11)                                                           \
+        _(t3) /* temporary register */                                   \
+        _(t4)                                                            \
+        _(t5)                                                            \
+        _(t6))
+/* clang-format on */
 
 /* RISC-V registers (mnemonics, ABI names)
  *
@@ -117,6 +120,7 @@ enum SV32_PTE_PERM {
 #define MISA_SUPER (1 << ('S' - 'A'))
 #define MISA_USER (1 << ('U' - 'A'))
 #define MISA_I (1 << ('I' - 'A'))
+#define MISA_E (1 << ('E' - 'A'))
 #define MISA_M (1 << ('M' - 'A'))
 #define MISA_A (1 << ('A' - 'A'))
 #define MISA_F (1 << ('F' - 'A'))

src/syscall.c

@@ -466,8 +466,12 @@ static void syscall_sbi_rst(riscv_t *rv)
 
 void syscall_handler(riscv_t *rv)
 {
-    /* get the syscall number */
+/* get the syscall number */
+#if !RV32_HAS(RV32E)
     riscv_word_t syscall = rv_get_reg(rv, rv_reg_a7);
+#else
+    riscv_word_t syscall = rv_get_reg(rv, rv_reg_t0);
+#endif
 
     switch (syscall) { /* dispatch system call */
 #define _(name, number)     \