Bare metal VGA: ensure correct initial video mode, & obtain

initial cursor position as left by BIOS

libc/vga/vga-init.S

@@ -0,0 +1,66 @@
+/*-*- mode:unix-assembly; indent-tabs-mode:t; tab-width:8; coding:utf-8     -*-│
+│vi: set et ft=asm ts=8 tw=8 fenc=utf-8                                     :vi│
+╞══════════════════════════════════════════════════════════════════════════════╡
+│ This is free and unencumbered software released into the public domain.      │
+│                                                                              │
+│ Anyone is free to copy, modify, publish, use, compile, sell, or              │
+│ distribute this software, either in source code form or as a compiled        │
+│ binary, for any purpose, commercial or non-commercial, and by any            │
+│ means.                                                                       │
+│                                                                              │
+│ In jurisdictions that recognize copyright laws, the author or authors        │
+│ of this software dedicate any and all copyright interest in the              │
+│ software to the public domain. We make this dedication for the benefit       │
+│ of the public at large and to the detriment of our heirs and                 │
+│ successors. We intend this dedication to be an overt act of                  │
+│ relinquishment in perpetuity of all present and future rights to this        │
+│ software under copyright law.                                                │
+│                                                                              │
+│ THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,              │
+│ EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF           │
+│ MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.       │
+│ IN NO EVENT SHALL THE AUTHORS BE LIABLE FOR ANY CLAIM, DAMAGES OR            │
+│ OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE,        │
+│ ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR        │
+│ OTHER DEALINGS IN THE SOFTWARE.                                              │
+╚─────────────────────────────────────────────────────────────────────────────*/
+#include "libc/macros.internal.h"
+
+//	Code snippet for initializing the VGA video mode for bare metal.
+//
+//	If a program requests VGA support (by yoinking vga_console),
+//	and it is started in bare metal mode, then try to ensure that
+//	the VGA monitor is in a known mode.  This is easier to do while
+//	the program is still running in real mode.
+//
+//	This module also ropes in the sys_writev_vga routine, which
+//	implements the actual VGA console output under x86-64 long mode.
+//
+//	@see	rlinit & .sort.text.real.init.* (ape/ape.S)
+//	@see	ape/ape.lds
+//	@see	sys_writev_vga (libc/vga/writev-vga.c)
+	.section .sort.text.real.init.2,"ax",@progbits
+	.code16
+	mov	$0x4f03,%ax		# get current video mode via VESA
+	int	$0x10
+	cmp	$0x004f,%ax		# is VESA a thing here?
+	jz	1f
+	mov	$0x0f,%ah		# if not, get the video mode via a
+	int	$0x10			# classical BIOS call
+	cbtw
+	xchgw	%ax,%bx
+1:	mov	$0x0003,%ax		# check if we are in a 80 × ? × 16
+	cmp	%ax,%bx			# text mode
+	jnz	2f
+	cmpb	$25-1,0x0484		# check if number of screen rows
+	jnz	2f			# (BDA.ROWS + 1) is 25; if so, then
+	mov	$0x0500,%ax		# just make sure we are on display
+					# page 0
+2:	int	$0x10			# otherwise, change the video mode
+	.previous
+	.code64
+	.section .rodata,"a",@progbits
+vga_console:
+	.endobj	vga_console,globl,hidden
+	.previous
+	.yoink	sys_writev_vga

libc/vga/writev-vga.c

@@ -41,9 +41,7 @@ typedef struct {
 
 typedef char_cell_t char_row_t[WIDTH];
 
-const char vga_console[0];
+static unsigned short height = 25, curr_row, curr_col;
-
-static unsigned short height = 25, curr_row = 12, curr_col = 0;
 static uint8_t curr_attr = 0x07;
 
 static void scroll(void) {
@@ -71,8 +69,6 @@ static void updatexy_vga(void) {
 }
 
 static void writec_vga(char c) {
-  /* TODO: ensure screen in a known mode (text or graphics), at rlinit time */
-  /* TODO: use our own font, rather than rely on BIOS's CP437 font */
   /* TODO: handle UTF-8 multi-bytes */
   /* TODO: handle VT102 escape sequences */
   /* TODO: maybe make BEL (\a) character code emit an alarm of some sort */
@@ -127,3 +123,13 @@ ssize_t sys_writev_vga(struct Fd *fd, const struct iovec *iov, int iovlen) {
   updatexy_vga();
   return wrote;
 }
+
+__attribute__((__constructor__)) static textstartup void _vga_init(void) {
+  /* Get the initial cursor position from the BIOS data area. */
+  typedef struct {
+    unsigned char col, row;
+  } bios_curs_pos_t;
+  bios_curs_pos_t pos = *(bios_curs_pos_t *)(BANE + 0x0450ull);
+  curr_row = pos.row;
+  curr_col = pos.col;
+}