1 files changed, 73 insertions, 0 deletions
diff --git a/engine/flag_action.asm b/engine/flag_action.asm
new file mode 100644
index 00000000..dc516887
--- /dev/null
+++ b/engine/flag_action.asm
@@ -0,0 +1,73 @@
+FlagActionPredef:
+	call GetPredefRegisters
+
+FlagAction:
+; Perform action b on bit c
+; in the bitfield at hl.
+;  0: reset
+;  1: set
+;  2: read
+; Return the result in c.
+
+	push hl
+	push de
+	push bc
+
+	; bit
+	ld a, c
+	ld d, a
+	and 7
+	ld e, a
+
+	; byte
+	ld a, d
+	srl a
+	srl a
+	srl a
+	add l
+	ld l, a
+	jr nc, .ok
+	inc h
+.ok
+
+	; d = 1 << e (bitmask)
+	inc e
+	ld d, 1
+.shift
+	dec e
+	jr z, .shifted
+	sla d
+	jr .shift
+.shifted
+
+	ld a, b
+	and a
+	jr z, .reset
+	cp 2
+	jr z, .read
+
+.set
+	ld b, [hl]
+	ld a, d
+	or b
+	ld [hl], a
+	jr .done
+
+.reset
+	ld b, [hl]
+	ld a, d
+	xor $ff
+	and b
+	ld [hl], a
+	jr .done
+
+.read
+	ld b, [hl]
+	ld a, d
+	and b
+.done
+	pop bc
+	pop de
+	pop hl
+	ld c, a
+	ret