3 files changed, 45 insertions, 8 deletions
diff --git a/chapter21/README.md b/chapter21/README.md
index f2c74e07..a4efb00d 100644
--- a/chapter21/README.md
+++ b/chapter21/README.md
@@ -1,16 +1,51 @@
 # Chapter 21: Instruction Encoding and ELF
 
-This chapter has encodings for:
 
-*x86_64 v2*, 
-*RISC V 64I*
-*Arm V8*
+# Table of Contents
 
+1. [Instruction Encodings](#instruction-encodings)
+2. [AMD64](#amd64)
+3. [ARM](#arm)
+4. [RISCV](#riscv)
 
+This chapter will add instruction encodings.
+Instruction encoding refers to the representation of 
+machine instructions in a specific binary format defined by the 
+processor's instruction set architecture (ISA)
+
+After we established instruction selection, we need to encode the instructions before we can write them
+into an ELF object file.
+
+Currently, we support these architectures:
+### AMD64:
+For *x86-64(amd64):*, we use the latest Intel manual for the [encoding rules](https://www.felixcloutier.com/x86/).
+
+### RISC-V:
+
+For *riscv* we currently target [RVA23U64](https://msyksphinz-self.github.io/riscv-isadoc/html/).
+- ARM
+Each of which has its own instruction encoding format.
+
+To avoid ambiguity, we define the sources  of the information we use to encode the instructions:
+
+### ARMV8/AArch64 :
+For *arm*(aarch64) we use this collection for the [encoding rules](https://docsmirror.github.io/A64/2023-06/index.html).
+*Note*: We only support 64 bits ARM encodings.
+
+--- 
 ## Instruction Encodings
 
-## x86_64_v2
-*Source*: [^1] 
+
+## AMD64
+
+#### REX PREFIX
+#### MODR/M
+#### SIB
+#### Displacement
+#### Immediate
+#### Indirect
+
+=======
 
 ## ARM
 
diff --git a/chapter21/src/main/java/com/seaofnodes/simple/node/cpus/arm/FloatARM.java b/chapter21/src/main/java/com/seaofnodes/simple/node/cpus/arm/FloatARM.java
index 517d0831..2b089331 100644
--- a/chapter21/src/main/java/com/seaofnodes/simple/node/cpus/arm/FloatARM.java
+++ b/chapter21/src/main/java/com/seaofnodes/simple/node/cpus/arm/FloatARM.java
@@ -21,12 +21,13 @@ public class FloatARM extends ConstantNode implements MachNode {
     @Override public FloatARM copy() { return new FloatARM(this); }
 
     @Override public void encoding( Encoding enc ) {
+        enc.largeConstant(this,_con);
         short dst = (short)(enc.reg(this) - arm.D_OFFSET);
         double d = ((TypeFloat)_con).value();
         long x = Double.doubleToRawLongBits(d);
         // Any number that can be expressed as +/-n * 2-r,where n and r are integers, 16 <= n <= 31, 0 <= r <= 7.
-        //arm.f_mov(30,3,imm8,self);
-        throw Utils.TODO();
+        int body = arm.load_pc(0b01011100, 0, dst);
+        enc.add4(body);
     }
 
     // Human-readable form appended to the SB.  Things like the encoding,
diff --git a/chapter21/src/main/java/com/seaofnodes/simple/node/cpus/arm/arm.java b/chapter21/src/main/java/com/seaofnodes/simple/node/cpus/arm/arm.java
index 04569476..2ebcd781 100644
--- a/chapter21/src/main/java/com/seaofnodes/simple/node/cpus/arm/arm.java
+++ b/chapter21/src/main/java/com/seaofnodes/simple/node/cpus/arm/arm.java
@@ -277,6 +277,7 @@ public class arm extends Machine {
         enc.add4(body);
     }
 
+    // share same encoding with LDR (literal, SIMD&FP) flavour
     public static int load_pc(int opcode, int offset, int rt) {
         return (opcode << 24) | (offset << 5) | rt;
     }