Skip to content

Introduce local labels in hello-world.asm #154

New issue

Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community.

Sign up for GitHub

By clicking “Sign up for GitHub”, you agree to our terms of service and privacy statement. We’ll occasionally send you account related emails.

Already on GitHub? Sign in to your account

Merged
avivace merged 2 commits into from
Jan 15, 2026
Merged

Introduce local labels in hello-world.asm #154

Show file tree
Hide file tree
Changes from all commits
Commits
Show all changes
2 commits
Select commit Hold shift + click to select a range
7df2873
Introduce local labels in hello-world.asm
Rangi42 Jan 14, 2026
a6e2d51
There is no `.copyTiles` label
Rangi42 Jan 14, 2026
File filter

Filter by extension

Filter by extension
Conversations
Failed to load comments.
Loading
Jump to
Jump to file
Failed to load files.
Loading
Diff view
Diff view
12 changes: 8 additions & 4 deletions src/assets/hello-world.asm
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -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]
Expand All @@ -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
Expand All @@ -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
Expand Down Expand Up @@ -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
Expand All @@ -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
7 changes: 6 additions & 1 deletion src/part1/memory.md
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -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.
Expand Down Expand Up @@ -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!

---

Expand Down