Add MemPool's Xqueue atomic extension

README.md

@@ -33,6 +33,12 @@ starting from their high-level, human-readable description.
   (`opcodes-rvv` file) have been set as pseudo-instruction due to the overlapping
   of their opcodes space with the opcodes space of the SIMD instructions from
   Xpulpv2, defined in `opcodes-xpulpvect_CUSTOM` and `opcodes-xpulpvectshufflepack_CUSTOM`.
+- For quick reference: commonly in our projects, `encoding.h` is employed in
+  - the instruction decoder in the RTL: `riscv_instr.sv`
+  - bare-metal runtime
+  - `riscv-isa-sim`: `riscv-isa-sim/riscv/encoding.h`
+  - `riscv-tests`: `riscv-tests/env/encoding.h`
+  - `riscv-gnu-toolchain`: `riscv-binutils-gdb/include/opcode/riscv-opc.h`
 
 
 ## Smallfloat notice

config.mk

@@ -12,5 +12,5 @@ RV32XPULPIMG += opcodes-xpulpbitop_CUSTOM
 SNITCH_OPCODES := opcodes-dma_CUSTOM opcodes-frep_CUSTOM opcodes-ssr_CUSTOM
 
 # default configurations
-MEMPOOL_ISA := opcodes-frep_CUSTOM $(RV32XPULPIMG) opcodes-xpulppostmod_CUSTOM opcodes-rv32d-zfh_DRAFT opcodes-rv32q-zfh_DRAFT opcodes-rv32zfh_DRAFT opcodes-rv64zfh_DRAFT opcodes-sflt_CUSTOM
+MEMPOOL_ISA := opcodes-frep_CUSTOM $(RV32XPULPIMG) opcodes-xpulppostmod_CUSTOM opcodes-rv32d-zfh_DRAFT opcodes-rv32q-zfh_DRAFT opcodes-rv32zfh_DRAFT opcodes-rv64zfh_DRAFT opcodes-sflt_CUSTOM opcodes-xqueue_CUSTOM
 SNITCH_ISA := $(RV32XPULPIMG) $(SNITCH_OPCODES) opcodes-sflt_CUSTOM

opcodes-xqueue_CUSTOM

@@ -0,0 +1,15 @@
+# format of a line in this file:
+# <instruction name> <args> <opcode>
+#
+# <opcode> is given by specifying one or more range/value pairs:
+# hi..lo=value or bit=value or arg=value (e.g. 6..2=0x45 10=1 rd=0)
+#
+# <args> is one of rd, rs1, rs2, rs3, imm20, imm12, imm12lo, imm12hi,
+# shamtw, shamt, rm
+
+# Xqueue extension (atomic queue operations)
+# IMPORTANT: THIS IS AN ILLEGAL EXTENSION OF THE STANDARD ATOMIC INSTRUCTIONS
+
+# queue push and pop (32-bit)
+q.push   rd rs1 rs2      aqrl 31..29=1 28..27=3 14..12=2 6..2=0x0B 1..0=3
+q.pop    rd rs1 24..20=0 aqrl 31..29=1 28..27=2 14..12=2 6..2=0x0B 1..0=3