fix screen size and comment draw code

2 files changed, 8 insertions, 1 deletions

diff --git a/chip8.c b/chip8.c
index 821d75f..b79bc3a 100644
--- a/chip8.c
+++ b/chip8.c
@@ -197,6 +197,8 @@ void vm_step() {
       vm.Vx[0xF] = 0;
       for (u32 i = 0; i < (in&0xF); i++) {
         for (u32 s = 0; s < 8; s++) {
+
+          // 0x80 = 10000000
           u8 pix = (vm.ram[vm.I + i] & (0x80 >> s)) != 0;
 
           if (pix) {
@@ -204,8 +206,13 @@ void vm_step() {
             u8 x = (vm.Vx[in>>8&0xF] + s)&0x3F;
  
             u32 byte = y * 64 + x;
+
+            // put the single 1 bit in the position
+            // that is relative to the pixel
             u32 bit = 1 << (byte & 0x07);
 
+            // byte>>3 ~ byte / 8 
+            // used to get the index of byte to xor
             if (vm.screen[byte>>3] & bit) {
               vm.Vx[0xF] = 1;
             }
diff --git a/chip8.h b/chip8.h
index 2c7eb45..ce02394 100644
--- a/chip8.h
+++ b/chip8.h
@@ -23,7 +23,7 @@ typedef struct {
 
   u8 keyboard[16];
 
-  u8 screen[0x3F*0x1F];
+  u8 screen[250];
 } chip8_vm;
 
 extern chip8_vm vm;