Adding ModR/M byte macro ...

1 files changed, 16 insertions, 2 deletions

diff --git a/instructions.h b/instructions.h
index 9727137..1623ff0 100644
--- a/instructions.h
+++ b/instructions.h
@@ -47,6 +47,20 @@
 #define MOVABS(reg, imm) 0x48, MOVD(reg, imm)
 #define MOVQ(reg, imm)   0x48, 89 c8
 
+#define MODRM(mod, reg, rm) (((mod & 3) << 6) | ((reg & 7) << 3) | (rm & 3))
+// MOD   REG     RM  (r/m = register/memory)
+// 1 1 | 1 0 1 | 1 0 1
+// if mod == 0b11, then treat r/m as register,
+// (examples in OP SRC DST ordering (same as the reg-rm order)
+// gas/at&t ASM        => op  reg-idx reg-idx => MOD | REG   | RM
+// --------------------+----------------------+---------------------
+// ie "xor %rcx, %rcx" => XOR 1       1       => 1 1 | 0 0 1 | 0 0 1
+// ie "xor %rsi, %rbp" => XOR 6       5       => 1 1 | 1 0 1 | 1 1 0
+//
+// Other combinations of MOD means RM specifies an addressing mode.
+// For 64bit systems:
+//
+
 #define ADD
 #define CALL
 #define CMP
@@ -64,10 +78,10 @@
 #define LEA
 #define POP
 #define PUSH
-#define RET
+#define RET    0xc3
 #define RETN
 #define SUB
-#define XOR
+#define XOR /*TODO*/
 
 #define SYSCALL 0x0f, 0x05