Add doc about reg

Readme.md

@@ -14,6 +14,28 @@ Debug with
   b *0x7c00
   c
 
+== ASM reg ==
+
+=== In 16 bits ===
+There is 4 main 16bits reg: AX, BX, CX, DX.
+You can access the 8 MSB from AX with AH and the 8 LSB with AL.( BH,BL, CH, CL)
+
+SI and DI are 16bits index register often used for pointer but can contains data.
+
+SP is stack pointer, BP is base pointer
+
+AX(AH, AL), BX(BH, BL), CX(CH, CL), DX(DH, DL), SI, DI, SP, BP
+
+=== In 32bits ===
+Main register are "Extended" to 32bits but 16bits of 8bits direct access is not possible anymore.
+
+EAX, EBX, ECX, EDX, ESI, EDI, ESP, EBP
+
+=== In 64bits ===
+Prefixed by R
+RAX, RBX, RCX, RDX, RSI, RDI, RSP, RBP
+
+
 == Bios Memory ==
 
 0x0     -   0x03FF      ;IVT