Add explanation about 16-bit inc/dec

1 files changed, 34 insertions, 16 deletions

diff --git a/Optimizing-assembly-code.md b/Optimizing-assembly-code.md
index 35fa835..d5f60c5 100644
--- a/Optimizing-assembly-code.md
+++ b/Optimizing-assembly-code.md
@@ -11,12 +11,13 @@ Most of these tricks come from either [Jeff's GB Assembly Code Tips v1.0](http:/
   - [Invert the bits of `a`](#invert-the-bits-of-a)
   - [Multiply `hl` by 2](#multiply-hl-by-2)
   - [Add `a` to a 16-bit register](#add-a-to-a-16-bit-register)
-  - [Loading from an address to `hl`](#loading-from-an-address-to-hl)
-  - [Exchanging two 16-bit registers](#exchanging-two-16-bit-registers)
-  - [Loading a constant into `[hl]`](#loading-a-constant-into-hl)
-  - [Loading two constants into a register pair](#loading-two-constants-into-a-register-pair)
-  - [Loading a constant into `[hl]` and incrementing or decrementing `hl`](#loading-a-constant-into-hl-and-incrementing-or-decrementing-hl)
-  - [Incrementing or decrementing `[hl]`](#incrementing-or-decrementing-hl)
+  - [Increment a 16-bit register](#increment-decrement-a-16-bit-register)
+  - [Load from an address to `hl`](#load-from-an-address-to-hl)
+  - [Exchange two 16-bit registers](#exchange-two-16-bit-registers)
+  - [Load a constant into `[hl]`](#load-a-constant-into-hl)
+  - [Load two constants into a register pair](#load-two-constants-into-a-register-pair)
+  - [Load a constant into `[hl]` and increment or decrement `hl`](#load-a-constant-into-hl-and-increment-or-decrement-hl)
+  - [Increment or decrement `[hl]`](#increment-or-decrement-hl)
 - [Branching (control flow)](#branching-control-flow)
   - [Relative jumps](#relative-jumps)
   - [Compare `a` to 0](#compare-a-to-0)
@@ -25,11 +26,11 @@ Most of these tricks come from either [Jeff's GB Assembly Code Tips v1.0](http:/
   - [Add `a` to `hl` without using a 16-bit register](#add-a-to-hl-without-using-a-16-bit-register)
 - [Subroutines (functions)](#subroutines-functions)
   - [Tail call optimization](#tail-call-optimization)
-  - [Calling `hl`](#calling-hl)
+  - [Call `hl`](#call-hl)
   - [Inlining](#inlining)
   - [Fallthrough](#fallthrough)
 - [Jump and lookup tables](#jump-and-lookup-tables)
-  - [Chaining comparisons](#chaining-comparisons)
+  - [Chain comparisons](#chaining-comparisons)
 
 
 ## Registers
@@ -170,7 +171,24 @@ Or better (doesn't require a label):
 ```
 
 
-### Loading from an address to `hl`
+### Increment / decrement a 16-bit register
+
+When possible, avoid doing:
+
+```asm
+	inc hl ; 1 byte, 2 cycles
+```
+
+If you can ensure that the low byte won't overflow (or that it won't matter if it does), then do this:
+
+```asm
+	inc l ; 1 byte, 1 cycle
+```
+
+Further, if you intend your code to run on DMG (black & white GB), avoiding 16-bit inc/dec means less occasions to trigger the OAM corruption bug.
+
+
+### Load from an address to `hl`
 
 Don't do:
 
@@ -193,7 +211,7 @@ But do:
 ```
 
 
-### Exchanging two 16-bit registers
+### Exchange two 16-bit registers
 
 (The example uses `hl` and `de`, but any pair of `bc`, `de`, or `hl` would also work.)
 
@@ -220,7 +238,7 @@ If you care about size:
 ```
 
 
-### Loading a constant into `[hl]`
+### Load a constant into `[hl]`
 
 Don't do:
 
@@ -238,7 +256,7 @@ But do:
 ```
 
 
-### Loading two constants into a register pair
+### Load two constants into a register pair
 
 (The example uses `bc`, but `hl` or `de` would also work.)
 
@@ -265,7 +283,7 @@ Or better, use the `lb` macro in [macros/code.asm](../blob/master/macros/code.as
 ```
 
 
-### Loading a constant into `[hl]` and incrementing or decrementing `hl`
+### Load a constant into `[hl]` and incrementing or decrementing `hl`
 
 Don't do:
 
@@ -298,7 +316,7 @@ But do:
 ```
 
 
-### Incrementing or decrementing `[hl]`
+### Increment or decrement `[hl]`
 
 Don't do:
 
@@ -477,7 +495,7 @@ But do:
 ```
 
 
-### Calling `hl`
+### Call `hl`
 
 ```asm
 	; 5 bytes, 8 cycles
@@ -558,7 +576,7 @@ You can still `call Function` elsewhere, but one tail call can be optimized into
 ## Jump and lookup tables
 
 
-### Chaining comparisons
+### Chain comparisons
 
 Don't do: