diff --git a/model/riscv_types.sail b/model/riscv_types.sail
index e9a5a1990..b312095ba 100644
--- a/model/riscv_types.sail
+++ b/model/riscv_types.sail
@@ -333,7 +333,7 @@ enum ropw  = {RISCV_ADDW, RISCV_SUBW, RISCV_SLLW,
 enum sopw = {RISCV_SLLIW, RISCV_SRLIW,
              RISCV_SRAIW}                         /* RV64-only shift ops */
 enum amoop = {AMOSWAP, AMOADD, AMOXOR, AMOAND, AMOOR,
-              AMOMIN, AMOMAX, AMOMINU, AMOMAXU}   /* AMO ops */
+              AMOMIN, AMOMAX, AMOMINU, AMOMAXU, AMOCAS}   /* AMO ops */
 enum csrop = {CSRRW, CSRRS, CSRRC}                /* CSR ops */
 
 enum brop_zba = {RISCV_SH1ADD, RISCV_SH2ADD, RISCV_SH3ADD}
@@ -389,6 +389,33 @@ mapping size_bytes : word_width <-> {1, 2, 4, 8} = {
   DOUBLE <-> 8,
 }
 
+enum amo_word_width = {AMO_BYTE, AMO_HALF, AMO_WORD, AMO_DOUBLE, AMO_QUAD}
+
+mapping amo_size_enc : amo_word_width <-> bits(3) = {
+  AMO_BYTE   <-> 0b000,
+  AMO_HALF   <-> 0b001,
+  AMO_WORD   <-> 0b010,
+  AMO_DOUBLE <-> 0b011,
+  AMO_QUAD   <-> 0b100,
+}
+
+mapping amo_size_mnemonic : amo_word_width <-> string = {
+  AMO_BYTE   <-> "b",
+  AMO_HALF   <-> "h",
+  AMO_WORD   <-> "w",
+  AMO_DOUBLE <-> "d",
+  AMO_QUAD   <-> "q",
+}
+
+val amo_word_width_bytes : amo_word_width -> {'s, 's == 1 | 's == 2 | 's == 4 | 's == 8 | 's == 16 . atom('s)}
+function amo_word_width_bytes width = match width {
+  AMO_BYTE   => 1,
+  AMO_HALF   => 2,
+  AMO_WORD   => 4,
+  AMO_DOUBLE => 8,
+  AMO_QUAD   => 16
+}
+
 /*!
  * Raise an internal error reporting that width w is invalid for access kind, k,
  * and current xlen. The file name and line number should be passed in as the