[Reverse-engineering] [th03] CDG: VRAM plane iteration

Which is where ZUN adds those additional 0x2000 to make the jump from
the 0xB800 plane to the 0xE000 plane.

Part of P0114, funded by Lmocinemod.

pc98.h

@@ -114,6 +114,11 @@ typedef int8_t uint4_t;
 #define SEG_PLANE_R 0xB000
 #define SEG_PLANE_G 0xB800
 #define SEG_PLANE_E 0xE000
+
+// Segment distance between B↔R↔G
+#define SEG_PLANE_DIST_BRG 0x800
+// Segment distance between G↔E
+#define SEG_PLANE_DIST_E 0x2800
 /// ---------------
 
 /// Keyboard

pc98.inc

@@ -19,3 +19,10 @@ ROW_SIZE = (RES_X / 8)
 PLANE_SIZE = (ROW_SIZE * RES_Y)
 COLOR_COUNT = 16
 ; ========
+
+SEG_PLANE_B = 0A800h
+SEG_PLANE_R = 0B000h
+SEG_PLANE_G = 0B800h
+SEG_PLANE_E = 0E000h
+SEG_PLANE_DIST_BRG = 800h
+SEG_PLANE_DIST_E = 2800h

th03/formats/cdg.inc

@@ -21,3 +21,25 @@ cdg_t struc
 	seg_alpha dw ?
 	seg_colors dw ?
 cdg_t ends
+
+vram_plane_next macro seg:req, label_loop:req
+	; Next segment...
+	mov	ax, seg
+	add	ax, SEG_PLANE_DIST_BRG
+	mov	seg, ax
+
+	; Still in the R or G segment?
+	cmp	ax, (SEG_PLANE_G + SEG_PLANE_DIST_BRG)
+	jb	short label_loop
+
+	; Already in the E segment?
+	cmp	ax, (SEG_PLANE_G + (SEG_PLANE_DIST_BRG * 2))
+	jnb	short @@vram_plane_next_done
+
+	; Make the large jump between G and E
+	add	ax, (SEG_PLANE_DIST_E - SEG_PLANE_DIST_BRG)
+	mov	seg, ax
+	jmp	short label_loop
+
+@@vram_plane_next_done:
+endm

th03/formats/cdg_put.asm

@@ -63,6 +63,7 @@ cdg_put proc far
 	mov	ds, ax
 	assume ds:nothing
 
+@@row_loop:
 @@width_2:
 	mov	cx, 1234h
 
@@ -73,20 +74,9 @@ cdg_put proc far
 	add	di, 4
 	loop	@@blit_dword
 	sub	di, dx
-	jns	short @@width_2
+	jns	short @@row_loop
 	mov	di, bx
-	mov	ax, es
-	add	ax, 800h
-	mov	es, ax
-	assume es:nothing
-	cmp	ax, 0C000h
-	jb	short @@width_2
-	cmp	ax, 0C800h
-	jnb	short @@ret
-	add	ax, 2000h
-	mov	es, ax
-	assume es:nothing
-	jmp	short @@width_2
+	vram_plane_next es, @@row_loop
 
 @@ret:
 	pop	ds

th03/formats/cdg_put_hflip.asm

@@ -75,6 +75,7 @@ proc_defconv cdg_put_hflip
 	mov	dx, 1234h
 	nop
 
+@@row_loop:
 @@width_bytes_colors:
 	mov	cx, 1234
 
@@ -86,22 +87,11 @@ proc_defconv cdg_put_hflip
 	dec	di
 	loop	@@blit_hflip_colors
 	sub	di, dx
-	jns	short @@width_bytes_colors
+	jns	short @@row_loop
 
 @@last_x_colors:
 	mov	di, 1234h
-	mov	ax, es
-	add	ax, 800h
-	mov	es, ax
-	assume es:nothing
-	cmp	ax, 0C000h
-	jb	short @@width_bytes_colors
-	cmp	ax, 0C800h
-	jnb	short @@ret
-	add	ax, 2000h
-	mov	es, ax
-	assume es:nothing
-	jmp	short @@width_bytes_colors
+	vram_plane_next es, @@row_loop
 
 @@ret:
 	pop	di

th03/formats/cdg_put_noalpha.asm

@@ -40,24 +40,14 @@ cdg_put_noalpha	proc far
 	cld
 	nop
 
+@@row_loop:
 @@width:
 	mov	cx, 1234h
 	rep movsd
 	sub	di, dx
-	jns	short @@width
+	jns	short @@row_loop
 	mov	di, bx
-	mov	ax, es
-	add	ax, 800h
-	mov	es, ax
-	assume es:nothing
-	cmp	ax, 0C000h
-	jb	short @@width
-	cmp	ax, 0C800h
-	jnb	short @@ret
-	add	ax, 2000h
-	mov	es, ax
-	assume es:nothing
-	jmp	short @@width
+	vram_plane_next es, @@row_loop
 
 @@ret:
 	pop	ds