Implement draw text functionality

include/apple2.draw.h

@@ -0,0 +1,7 @@
+#ifndef _APPLE2_DRAW_H_
+#define _APPLE2_DRAW_H_
+
+extern void apple2_draw_pixel(apple2 *, vm_16bit);
+extern void apple2_draw_text(apple2 *, vm_16bit);
+
+#endif

sources.cmake

@@ -1,6 +1,7 @@
 set(erc_sources
 	apple2.c
 	apple2.dd.c
+	apple2.draw.c
 	log.c
 	mos6502.c
 	mos6502.addr.c

src/apple2.c

@@ -6,6 +6,7 @@
  */
 
 #include "apple2.h"
+#include "apple2.draw.h"
 #include "option.h"
 #include "vm_segment.h"
 
@@ -86,21 +87,10 @@ apple2_set_color(apple2 *mach, int mode)
     // correct color interpretation
 }
 
-#define SHOW(ch) \
-    vm_bitfont_render(mach->sysfont, mach->screen, &area, ch); \
-    area.x += 7
-
 void
 apple2_run_loop(apple2 *mach)
 {
-    SDL_Rect area;
-
-
     while (vm_screen_active(mach->screen)) {
-        area.x = 50;
-        area.y = 50;
-        SHOW('H'); SHOW('e'); SHOW('l'); SHOW('l'); SHOW('o'); SHOW(' ');
-        SHOW('w'); SHOW('o'); SHOW('r'); SHOW('l'); SHOW('d'); SHOW('!');
         vm_screen_refresh(mach->screen);
     }
 }

src/apple2.draw.c

@@ -2,15 +2,82 @@
  * apple2.draw.c
  */
 
+#include "apple2.h"
+
 void
 apple2_draw_pixel(apple2 *mach, vm_16bit addr)
 {
 }
 
+// Drawing text is a two-step process; first you need to wipe out
+// the block on the screen at that point. Then you need to draw the
+// text in over that.
+
 void
 apple2_draw_text(apple2 *mach, vm_16bit addr)
 {
-    // Drawing text is a two-step process; first you need to wipe out
-    // the block on the screen at that point. Then you need to draw the
-    // text in over that.
+    vm_8bit lsb, msb;
+    vm_16bit page_base;
+    SDL_Rect dest;
+    char ch;
+
+    // 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;
+    }
+
+    // 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.
+    msb = (addr >> 8) & 0xff;   // row
+    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 > 39) {
+        return;
+    }
+
+    if ((addr & 0xff80) % 128 != 0) {
+        return;
+    }
+
+    // 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.
+    dest.x = lsb * mach->sysfont->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.
+    dest.y = ((addr & 0xff80) - page_base) * mach->sysfont->height;
+
+    // Our width and height must be that of the font.
+    dest.w = mach->sysfont->width;
+    dest.h = mach->sysfont->height;
+
+    // And...lastly...what's in the address?
+    ch = (char)vm_segment_get(mach->memory, addr);
+
+    // Let's firstly blank out that space on screen.
+    //vm_bitfont_render(mach->sysfont, mach->screen, &dest, ' ');
+
+    // Now show the goddamned thing
+    vm_bitfont_render(mach->sysfont, mach->screen, &dest, ch);
 }