Only the top portion of the screen become black when any key is pressed

[Md. Al-Imran Abir]

I have run the code as described in the book. However, when I press a key only a portion (approx 1/15) of the screen blackens. I tried to figure it out but failed. Here is the pseudo-code:


  scradr = SCREEN
  row = 256
  col = 32

INFINITE_LOOP:
  i = 0
  k = KBD
  if k == 0 goto NOT_PRESSED

  PRESSED:
    if i >= row goto INFINITE_LOOP
    j = 0
    PRESSED_INNER_LOOP:
      if j >= col goto BREAK_INNER_LOOP
      RAM[scradr+i+j] = -1
      j = j + 1
      goto PRESSED_INNER_LOOP

    BREAK_INNER_LOOP:
      i = i + 1
      goto PRESSED

  NOT_PRESSED:
    if i >= row goto INFINITE_LOOP
    j = 0
    NOT_PRESSED_INNER_LOOP:
      if j >= col goto BREAK_INNER_LOOP_2
      RAM[scradr+i+j] = 0
      j = j + 1
      goto NOT_PRESSED_INNER_LOOP

    BREAK_INNER_LOOP_2:
      i = i + 1
      goto NOT_PRESSED

Can someone please say how can this be solved?

[Md. Al-Imran Abir]

I have been able to solve it so no need to bother. There was a logical error in the pseudo-code.

[WBahn]

Glad you found it.

Was the error neglecting to multiply the row index by the number of words in a row?

[WBahn]

Look at the difference between your code segments for pressed and unpressed. What do they differ by? What would it take to combine them into a single code segment? There are a couple of ways to do it.

[Md. Al-Imran Abir]

Yes, you have got it right.

[Md. Al-Imran Abir]

I think a more efficient pseudo-code maybe like this

scradr = SCREEN
row = 256
col = 32

INFINITE_LOOP:

i = 0
k = KBD

for(i = 0; i < row*col; i++)
     if k == 0
         RAM[scradr+i] = 0
     else
         RAM[scradr+i] = 1
end of for

goto INFINITE_LOOP