Add x86 loop instruction callout

Signed-off-by: Malfurious <m@lfurio.us>

1 files changed, 15 insertions, 0 deletions

diff --git a/docs/re/arch_x86.txt b/docs/re/arch_x86.txt
index f1f2a03..85cf22f 100644
--- a/docs/re/arch_x86.txt
+++ b/docs/re/arch_x86.txt
@@ -150,3 +150,18 @@ for(i = 0x20; i != 0; i--)
     *buf_ptr = 0;
     buf_ptr++;
 ```
+
+
+LOOP instruction
+----------------
+#from stack overflow:
+#https://stackoverflow.com/questions/46881279/how-exactly-does-the-x86-loop-instruction-work
+
+LOOP is exactly like `dec ecx / jnz`, except it doesn't set flags.
+
+It's like the bottom of a `do {} while (--ecx != 0);` loop in C.  If execution
+enters the loop with ecx=0, wrap-around means the loop will run 2**32 times
+(2**64 times in 64-bit mode).
+
+Unlike `rep movsb/stosb/etc`, it doesn't check for ecx=0 before decrementing,
+only after.