add resources to README

twasm/README.md

@@ -6,6 +6,11 @@ this will be a self hosted, very minimal subset of nasm-style 64 bit asm
 
 I want to compile Bootler and Twasm with the Twasm assembler
 
+### reading
+
+- [instructions](https://www.felixcloutier.com/x86/)
+- [opcodes,ModR/M,SIB](http://ref.x86asm.net/coder64.html) (no secure site available)
+
 ### memory map
 
 ```
@@ -197,3 +202,30 @@ hlt
 0xFE0A ; newline
 0xFE00 ; null terminator
 ```
+
+#### nasm output with the above example program, bits 64
+
+```nasm
+0x31 ; XOR r/m16/32/64 r16/32/64
+0xC0 ; ModR/M byte
+     ; mod 11b  ; directly address the following:
+     ; reg 000b ; EAX
+     ; r/m 000b ; EAX
+
+0x48 ; 64 Bit Operand Size prefix
+0xFF ; with `reg` from ModR/M byte 000b:
+     ; INC r/m16/32/64
+0xC0 ; ModR/M byte
+     ; mod 11b  ; direct addressing
+     ; reg 000b ; RAX
+     ; r/m 000b ; RAX
+
+0x48 ; 64 Bit Operand Size prefix
+0x89 ; MOV r/m16/32/64 r16/32/64
+0x10 ; ModR/M byte
+     ; mod 00b  ; indirect addressing, no displacement
+     ; reg 010b ; RDX
+     ; r/m 000b ; [RAX]
+
+0xF4 ; HLT
+```