diff --git a/demo_blank.h b/demo_blank.h
index 7ab2f54..0686bb6 100644
--- a/demo_blank.h
+++ b/demo_blank.h
@@ -1,11 +1,11 @@
 // TODO make it static once KickC bug is fixed
-byte blank = 0;
+byte blank = BLANK_OFF;
 
 void flip_blank() {
    blank ^= 1;
 
-   if(blank) woz_puts("NORMAL\r");
-   else      woz_puts("BLANK\r");
+   if(blank == BLANK_OFF) woz_puts("NORMAL\r");
+   else                   woz_puts("BLANK\r");
 
    // write "blank" bit
    tms_blank(blank);
diff --git a/demo_interrupt.h b/demo_interrupt.h
index f7e1bca..1c7c14f 100644
--- a/demo_interrupt.h
+++ b/demo_interrupt.h
@@ -13,7 +13,7 @@ void demo_interrupt() {
    install_interrupt();
 
    // enables interrupts on the TMS9918
-   tms_interrupt(1);
+   tms_set_interrupt_bit(INTERRUPT_ENABLED);
 
    woz_puts("INTERRUPT INSTALLED\r");
    woz_puts("0 TURNS OFF\r");
@@ -21,11 +21,10 @@ void demo_interrupt() {
    woz_puts("E EXIT TO MAIN MENU\r");
 
    for(;;) {
-      if(keypressed()) {
       if(woz_iskeypressed()) {
          byte k = woz_getkey();
-              if(k=='1') { tms_interrupt(1); woz_puts("INT ENABLED\r"); }
-         else if(k=='0') { tms_interrupt(0); woz_puts("INT DISABLED\r"); }
+              if(k=='1') { tms_set_interrupt_bit(INTERRUPT_ENABLED);  woz_puts("INT ENABLED\r"); }
+         else if(k=='0') { tms_set_interrupt_bit(INTERRUPT_DISABLED); woz_puts("INT DISABLED\r"); }
          else if(k=='E') break;
       }
 
@@ -39,7 +38,7 @@ void demo_interrupt() {
    }
 
    // disables interrupts on the TMS9918
-   tms_interrupt(0);
+   tms_set_interrupt_bit(INTERRUPT_DISABLED);
 
    woz_puts("INTERRUPT STOPPED\r");
 }
diff --git a/tms9918.h b/tms9918.h
index c41c020..73c1d45 100644
--- a/tms9918.h
+++ b/tms9918.h
@@ -14,7 +14,7 @@ const byte HIADDRESS_MASK = 0b00111111;   // bit mask for the high byte of the a
 const byte WRITE_TO_REG   = 0b10000000;   // write to register command
 const byte REGNUM_MASK    = 0b00000111;   // bit mask for register number (0-7)
 
-// register 1 masks
+// register 0 masks
 const byte REG0_M3_MASK     = 0b00000010;
 const byte REG0_EXTVID_MASK = 0b00000001;
 
@@ -45,6 +45,7 @@ const byte COLOR_MAGENTA      = 0xD;
 const byte COLOR_GRAY         = 0xE;
 const byte COLOR_WHITE        = 0xF;
 
+// macro for combining foreground and background into a single byte value
 #define COLOR_BYTE(f,b)     (((f)<<4)|(b))
 
 // status register bits (read only)
@@ -93,21 +94,27 @@ void TMS_INIT(byte *table) {
    }
 }
 
-// turn on off interrupt enable bit on register 1
-void tms_interrupt(byte val) {
+const byte INTERRUPT_ENABLED  = 1;
+const byte INTERRUPT_DISABLED = 0;
+
+// sets the interrupt enable bit on register 1
+void tms_set_interrupt_bit(byte val) {
    byte regvalue = TMS_REGS_LATCH[1] & (~REG1_IE_MASK);
    if(val) regvalue |= REG1_IE_MASK;
    TMS_WRITE_REG(1, regvalue);
 }
 
-// turn on off blank bit on register 1
+const byte BLANK_ON  = 0;
+const byte BLANK_OFF = 1;
+
+// sets the blank bit on register 1
 void tms_blank(byte val) {
    byte regvalue = TMS_REGS_LATCH[1] & (~REG1_BLANK_MASK);
    if(val) regvalue |= REG1_BLANK_MASK;
    TMS_WRITE_REG(1, regvalue);
 }
 
-// turn on off external video bit on register 0
+// sets the external video input bit on register 0
 void tms_external_video(byte val) {
    byte regvalue = TMS_REGS_LATCH[0] & (~REG0_EXTVID_MASK);
    if(val) regvalue |= REG0_EXTVID_MASK;