Introduce local labels in hello-world.asm (#154)

* Introduce local labels in hello-world.asm

* There is no `.copyTiles` label

Author: Rangi42

src/assets/hello-world.asm

@@ -29,7 +29,7 @@ WaitVBlank:
 	; Copy the tile data
 	ld de, Tiles
 	ld hl, $9000
-	ld bc, TilesEnd - Tiles
+	ld bc, Tiles.End - Tiles
 CopyTiles:
 ; ANCHOR: memcpy_first_two
 	ld a, [de]
@@ -45,7 +45,7 @@ CopyTiles:
 	; Copy the tilemap
 	ld de, Tilemap
 	ld hl, $9800
-	ld bc, TilemapEnd - Tilemap
+	ld bc, Tilemap.End - Tilemap
 CopyTilemap:
 	ld a, [de]
 	ld [hli], a
@@ -71,6 +71,7 @@ Done:
 
 SECTION "Tile data", ROM0
 
+; ANCHOR: tiles
 Tiles:
 	db $00,$ff, $00,$ff, $00,$ff, $00,$ff, $00,$ff, $00,$ff, $00,$ff, $00,$ff
 	db $00,$ff, $00,$80, $00,$80, $00,$80, $00,$80, $00,$80, $00,$80, $00,$80
@@ -142,10 +143,12 @@ Tiles:
 	db $54,$ff, $aa,$ff, $54,$ff, $aa,$ff, $54,$ff, $aa,$ff, $54,$ff, $00,$ff
 	db $15,$ff, $2a,$ff, $15,$ff, $0a,$ff, $15,$ff, $0a,$ff, $01,$ff, $00,$ff
 	db $01,$ff, $80,$ff, $01,$ff, $80,$ff, $01,$ff, $80,$ff, $01,$ff, $00,$ff
-TilesEnd:
+.End:
+; ANCHOR_END: tiles
 
 SECTION "Tilemap", ROM0
 
+; ANCHOR: tilemap
 Tilemap:
 	db $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00,  0,0,0,0,0,0,0,0,0,0,0,0
 	db $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00,  0,0,0,0,0,0,0,0,0,0,0,0
@@ -165,4 +168,5 @@ Tilemap:
 	db $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00,  0,0,0,0,0,0,0,0,0,0,0,0
 	db $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00,  0,0,0,0,0,0,0,0,0,0,0,0
 	db $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00, $00,  0,0,0,0,0,0,0,0,0,0,0,0
-TilemapEnd:
+.End:
+; ANCHOR_END: tilemap

src/part1/memory.md

@@ -95,6 +95,11 @@ Writing out a label's name is equivalent to writing the address of the byte it's
 For example, consider the `ld de, Tiles` at line {{#line_no_of "ld\s+de\s*,\s*Tiles" ../assets/hello-world.asm}}.
 `Tiles` (line {{#line_no_of "^\s*Tiles:" ../assets/hello-world.asm}}) is referring to the first byte of the tile data; if we assume that the tile data ends up being stored starting at $0193, then `ld de, Tiles` is equivalent to `ld de, $0193`!
 
+There's a shorthand for writing *local* labels: start their name with a dot `.` and it will implicitly be prefixed with the name of the most recent non-local label.
+For example, consider the two `.End:` labels at lines {{#line_no_of "^\.End:" ../assets/hello-world.asm:tiles}} and {{#line_no_of "^\.End:" ../assets/hello-world.asm:tilemap}}.
+The first one is implicitly defining "`Tiles.End:`" because of the non-local `Tiles:` label before it on line {{#line_no_of "^\s*Tiles:" ../assets/hello-world.asm}}, and the second one is defining "`Tilemap.End:`" because of the non-local `Tilemap:` label on line {{#line_no_of "^\s*Tilemap:" ../assets/hello-world.asm}}.
+Local labels are useful because as your project gets larger, you'll need more and more very similar names, and it would be tedious to have to make up unique prefixes for them everywhere.
+
 ## What's with the brackets?
 
 Right, we came into this because we wanted to know what the brackets in `ld a, [de]` mean.
@@ -128,7 +133,7 @@ So, if we look at the first two instructions of `CopyTiles`:
 ...we can see that we're copying the byte in memory *pointed to* by `de` (that is, whose address is contained in `de`) into the byte pointed to by `hl`.
 Here, `a` serves as temporary storage, since the CPU is unable to perform `ld [hl], [de]` directly.
 
-While we're at this, let's examine the rest of `.copyTiles` in the following lessons!
+While we're at this, let's examine the rest of `CopyTiles` in the following lessons!
 
 ---