1
0
mirror of https://github.com/pevans/erc-c.git synced 2024-11-28 11:51:06 +00:00
erc-c/src/apple2.text.c

110 lines
3.4 KiB
C
Raw Normal View History

/*
* apple2.text.c
*
* This code sits as a kind of wrapper over the bitmap fonts that the
* Apple II uses. Neither the system nor inverted fonts are exactly
* composed in the type of character set that Apple expects; they
* implement the glpyhs we need in order to make such. You can have
* inversed characters in the primary character set, for one; in the
* alternate character set, you can have MouseText glyphs, yet they are
* implemented as part of both fonts.
*/
#include "apple2.text.h"
void
apple2_text_draw(apple2 *mach, size_t addr)
{
int err;
vm_8bit ch;
vm_area dest;
vm_bitfont *font;
// If we're updating a page 2 address and we're not in some kind of
// double resolution mode, then we shouldn't actually render the
// thing.
if (addr > 0x07FF && !apple2_is_double_video(mach)) {
return;
}
// Default
font = mach->sysfont;
err = apple2_text_area(&dest, font, addr);
if (err != OK) {
return;
}
// What are we working with?
ch = mos6502_get(mach->cpu, addr);
// We treat special characters as spaces for display purposes.
if (ch < 0x20) {
vm_bitfont_render(font, mach->screen, &dest, ' ');
return;
}
// Blank out the space on the screen, then show the character
vm_bitfont_render(font, mach->screen, &dest, ' ');
vm_bitfont_render(font, mach->screen, &dest, ch);
}
int
apple2_text_area(vm_area *area, vm_bitfont *font, size_t addr)
{
vm_8bit lsb;
int page_base;
// The text display buffers are located at "Page 1" and "Page 2",
// which are at byte 1024-2047 (0x0400-0x07FF) and byte 2048-3071
// (0x0800-0x0BFF) respectively. If the given address is not in
// those (contiguous) ranges, then let's bail.
if (addr < 0x0400 || addr > 0x0BFF) {
return 0;
}
// In a given page for 40-column mode, you get 960 grid parts that
// you may use. In 80-column mode, it's more like 1920 grid parts
// (40x24 = 960, 80x24 = 1920). The way we look at this is the
// address indicates the place on the grid where text should go. We
// don't care how it got there. Let's figure out that position
// on-screen.
lsb = addr & 0xff; // column
// Regardless of which page we're rendering into, we can only use 40
// cells on the grid (that is, 0-39 from whatever value the msb is).
// It's possible to have an lsb greater than that, but if so, it's
// not anything we can render to the screen.
if (lsb > 0x39) {
return 0;
}
if ((addr & 0xff80) % 128 != 0) {
return 0;
}
// By default, we assume we're in text page 1. If the address ends
// up being greater than 0x07FF, then we must be in page 2.
page_base = 0x0400;
if (addr > 0x07FF) {
page_base = 0x0800;
}
// The absolute column position will be the font width times the
// lsb.
area->xoff = lsb * font->width;
// The absolute row position will be the font height times the msb
// minus the page base (because the height is the same regardless of
// what page we're in). So if we're msb $0400, then we're starting
// on pixel row 0; but if we're msb $0480, then we are starting on
// pixel row 8 (where the font height is 8); etc.
area->yoff = ((addr & 0xff80) - page_base) * font->height;
// Our width and height must be that of the font.
area->width = font->width;
area->height = font->height;
return OK;
}