Module Name: src
Committed By: tsutsui
Date: Sat May 4 16:06:57 UTC 2024
Modified Files:
src/sys/arch/hp300/dev: topcat.c
Log Message:
Add comments about quirks of 98542/98543 framebuffers with 1024x400 pixels.
To generate a diff of this commit:
cvs rdiff -u -r1.11 -r1.12 src/sys/arch/hp300/dev/topcat.c
Please note that diffs are not public domain; they are subject to the
copyright notices on the relevant files.
Modified files:
Index: src/sys/arch/hp300/dev/topcat.c
diff -u src/sys/arch/hp300/dev/topcat.c:1.11 src/sys/arch/hp300/dev/topcat.c:1.12
--- src/sys/arch/hp300/dev/topcat.c:1.11 Wed May 1 19:28:33 2024
+++ src/sys/arch/hp300/dev/topcat.c Sat May 4 16:06:57 2024
@@ -1,4 +1,4 @@
-/* $NetBSD: topcat.c,v 1.11 2024/05/01 19:28:33 tsutsui Exp $ */
+/* $NetBSD: topcat.c,v 1.12 2024/05/04 16:06:57 tsutsui Exp $ */
/* $OpenBSD: topcat.c,v 1.15 2006/08/11 18:33:13 miod Exp $ */
/*
@@ -351,11 +351,24 @@ topcat_reset(struct diofb *fb, int scode
/*
* Some displays, such as the HP332 and HP340 internal video
* and HP98542/98543 appear to return a display width of 1024
- * instead of 512.
+ * instead of 512. It looks these boards have actually have
+ * enough 64KB (1bpp) or 256KB (4bpp) VRAM and RAMDAC capabilities
+ * to display 1024x400 pixels.
*
- * It looks these boards have VRAM with sparse address layout,
- * i.e. 1 bit or 4 bits per pixel but 2 bytes per pixel, so
- * we have to handle 512 pixels per line with 1024 bytes per line.
+ * However HP's officlal "Service Information Manual" for
+ * "HP 900 Series 300 Computers Models 330/350" says:
+ * "The medium-resolution board uses eight memory chips per plane.
+ * This is enough to display 512 doubled pixels by 400 scan lines."
+ *
+ * This "512 doubled pixels" implies that the native HP-UX treated
+ * these 1024x400 framebuffers as pseudo 512x400 ones because
+ * ancient 1980s CRTs (such as 35741) didn't display such higher
+ * resolution. Furthermore, even modern LCDs can only handle
+ * upto 720 pixels in the "400 line" as VGA compatible mode.
+ *
+ * As mentioned above, we treat these 1024x400 1 bit or 4 bit
+ * framebuffers as "2 bytes per pixel" ones, so we have to handle
+ * 512 pixels per line with 1024 bytes per line.
*/
if (fb->planes <= 4 && fb->dwidth == 1024 && fb->dheight == 400) {
fb->dwidth = 512;
@@ -580,7 +593,8 @@ topcat_putchar8(void *cookie, int row, i
}
/*
- * Put a single character on 1 bpp or 4 bpp variants with sparse VRAM addresses
+ * Put a single character on 1 bpp (98542) or 4 bpp (98543) variants
+ * with 1024x400 VRAM to treat them as a pseudo 512x400 bitmap.
*/
static void
topcat_putchar1_4(void *cookie, int row, int col, u_int uc, long attr)
@@ -612,8 +626,8 @@ topcat_putchar1_4(void *cookie, int row,
tc_waitbusy(tc, diofb->planemask);
/*
- * At least HP98543 requires to write 4 bpp data onto
- * both odd and even addresses.
+ * We have to put pixel data to both odd and even addresses
+ * to handle "doubled pixels" as noted above.
*/
if (uc == ' ') {
uint16_t c = clr[0];
