Optimize 'sub hl, de'

1 files changed, 45 insertions, 1 deletions

diff --git a/Optimizing-assembly-code.md b/Optimizing-assembly-code.md
index 7aed42f..0c5e733 100644
--- a/Optimizing-assembly-code.md
+++ b/Optimizing-assembly-code.md
@@ -1,6 +1,6 @@
 Sometimes the simplest way to write something in assembly code isn't the best. All of your resources are limited: CPU speed, ROM size, RAM space, register use. You can rewrite code to use those resources more efficiently (sometimes by trading one for another).
 
-Most of these tricks come from [Jeff's GB Assembly Code Tips v1.0](http://www.devrs.com/gb/files/asmtips.txt), [WikiTI's Z80 Optimization page](http://wikiti.brandonw.net/index.php?title=Z80_Optimization), and [z80 Heaven's optimization tutorial](http://z80-heaven.wikidot.com/optimization). (Note that Z80 assembly is *not* the same as GBZ80; it has more registers and some different instructions.)
+Most of these tricks come from [Jeff's GB Assembly Code Tips v1.0](http://www.devrs.com/gb/files/asmtips.txt), [WikiTI's Z80 Optimization page](http://wikiti.brandonw.net/index.php?title=Z80_Optimization), [z80 Heaven's optimization tutorial](http://z80-heaven.wikidot.com/optimization), and [GBDev Wiki's ASM Snippets](https://gbdev.gg8.se/wiki/articles/ASM_Snippets). (Note that Z80 assembly is *not* the same as GBZ80; it has more registers and some different instructions.)
 
 WikiTI's advice fully applies here:
 
@@ -41,6 +41,7 @@ WikiTI's advice fully applies here:
   - [Add or subtract the carry flag from a 16-bit register](#add-or-subtract-the-carry-flag-from-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)
+  - [Subtract two 16-bit registers](#subtract-two-16-bit-registers)
   - [Load two constants into a register pair](#load-two-constants-into-a-register-pair)
   - [Load a constant into `[hl]`](#load-a-constant-into-hl)
   - [Increment or decrement `[hl]`](#increment-or-decrement-hl)
@@ -1258,6 +1259,49 @@ If you care about size:
 ```
 
 
+### Subtract two 16-bit registers
+
+(The example uses `hl` and `de`, but any pair of `bc`, `de`, or `hl` would also work.)
+
+Don't do:
+
+```asm
+	; 9 bytes, 10 cycles, modifies subtrahend de
+	ld a, $ff
+	xor d
+	ld d, a
+	ld a, $ff
+	xor e
+	ld e, a
+	add hl, de
+```
+
+And don't do:
+
+```asm
+	; 7 bytes, 8 cycles, modifies subtrahend de
+	ld a, d
+	cpl
+	ld d, a
+	ld a, e
+	cpl
+	ld e, a
+	add hl, de
+```
+
+But do:
+
+```asm
+	; 6 bytes, 6 cycles
+	ld a, l
+	sub e
+	ld l, a
+	ld a, h
+	sbc d
+	ld h, a
+```
+
+
 ### Load two constants into a register pair
 
 (The example uses `bc`, but `hl` or `de` would also work.)