shared: Add utilities for clearing screens

These are quite mindless, but they are rather useful on simple examples
in which we don't care about performance and we just want to show
something on the screen.

For now this didn't matter much, but on real hardware I was able to
reproduce the same glitches as with emulators which randomized memory on
both the CPU memory and on the PPU. Hence, just allow some examples to
manually reset these regions and avoid silly graphical glitches.

Signed-off-by: Miquel Sabaté Solà <mikisabate@gmail.com>

4 files changed, 50 insertions, 0 deletions

diff --git a/basics/flicker.s b/basics/flicker.s
index 234c8a1..8db03e8 100644
--- a/basics/flicker.s
+++ b/basics/flicker.s
@@ -21,8 +21,11 @@
 .segment "CODE"
 
 .include "../shared/diskun.s"
+.include "../shared/clear.s"
 
 .proc main
+    CLEAR_SCREEN
+
     jsr Diskun::init_palettes
     jsr init_sprites
 
diff --git a/fx/blink.s b/fx/blink.s
index 4fdaa6b..fb63ffd 100644
--- a/fx/blink.s
+++ b/fx/blink.s
@@ -122,6 +122,7 @@
 ;;; player around.
 .segment "FIXED"
 .include "../shared/diskun.s"
+.include "../shared/clear.s"
 
 ;;; NOTE: the main bulk of this example. Comments only for the parts which are
 ;;; specific to this example.
@@ -252,6 +253,8 @@ reset:
     lda #4
     sta Vars::last_bank
 
+    CLEAR_SCREEN
+
     jsr Diskun::init_palettes
     jsr init_sprites
 
diff --git a/scroll/roulette.s b/scroll/roulette.s
index 4b13c22..aca7d72 100644
--- a/scroll/roulette.s
+++ b/scroll/roulette.s
@@ -107,6 +107,7 @@
 ;;; player around.
 .segment "FIXED"
 .include "../shared/diskun.s"
+.include "../shared/clear.s"
 
 ;;; NOTE: the main bulk of this example. Comments only for the parts which are
 ;;; specific to this example.
@@ -201,6 +202,10 @@ reset:
 
 ;; The main function is used here only for further initialization purposes.
 .proc main
+    ;; Clear both screens to avoid funky business, as we are not doing anything
+    ;; specially clever here.
+    CLEAR_SCREENS $20, $24
+
     ;; Initialize both the palettes and the nametables.
     jsr Diskun::init_palettes
     jsr init_nametables
diff --git a/shared/clear.s b/shared/clear.s
new file mode 100644
index 0000000..a099387
--- /dev/null
+++ b/shared/clear.s
@@ -0,0 +1,39 @@
+;; Clear the first nametable by resetting it to the default tile ID. This is a
+;; rather drastic measure only to be done on examples that don't actually load
+;; columns in "clever" ways, but they just want a clean slate.
+.macro CLEAR_SCREEN
+    ldy #$20
+    jsr clear_screen_y
+.endmacro
+
+;; Clear the given two nametables. `ONE` and `OTHER` have to be the high byte
+;; for each nametable to clear. For example, on an horizontal scrolling game:
+;; `CLEAR_SCREENS $20, $24`.
+.macro CLEAR_SCREENS ONE, OTHER
+    ldy #ONE
+    jsr clear_screen_y
+
+    ldy #OTHER
+    jsr clear_screen_y
+.endmacro
+
+;; Clear the nametable with the high byte as defined in the `y` register.
+.proc clear_screen_y
+    bit $2002
+
+    ldx #$FF
+@loop:
+    sty $2006
+    stx $2006
+    lda #$00
+    sta $2007
+
+    dex
+    bne @loop
+
+    iny
+    cpy #$24
+    bne @loop
+
+    rts
+.endproc