Hoist segment register modification out of drawing subroutines

ponydos.asm

@@ -87,6 +87,12 @@ initialize_screen:
 	mov ch, 0x20
 	int 0x10
 
+	; Set up segments for drawing routines
+	xor ax, ax
+	mov ds, ax
+	push word 0xb800
+	pop es
+
 	call draw_wallpaper
 	call flip_mouse_cursor
 
@@ -115,33 +121,25 @@ mainloop:
 ; Drawing subroutines
 ; ------------------------------------------------------------------
 
+; requires:
+;  ds = 0
+;  es = 0xb800
 draw_wallpaper:
 	pusha
-	push es
-	push ds
-
-	xor ax, ax
-	mov ds, ax
-
-	push word 0xb800
-	pop es
 
 	mov si, WALLPAPER
 	xor di, di
 	mov cx, 80*25
 	rep movsw
 
-	pop ds
-	pop es
 	popa
 	ret
 
+; requires:
+;  ds = 0
+;  es = 0xb800
 flip_mouse_cursor:
 	pusha
-	push es
-
-	push word 0xb800
-	pop es
 
 	; Column
 	xor bh, bh
@@ -161,7 +159,6 @@ flip_mouse_cursor:
 	ror al, cl
 	mov [es:bx], al
 
-	pop es
 	popa
 	ret