Routines
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54639: Update Screen
Used by the routines at RunGame, GameOver and DisplayTextList.
Copies the screen buffer to the screen, and clears the buffer as it goes. The buffer holds the lines of the screen from the bottom up, 32 bytes to a line, so the lines are copied from the bottom of the screen to the top.
Before After UpdateScreen
update-screen-before update-screen-after
Copy each line from the buffer to the display, then clear the buffer.
UpdateScreen 54639 LD HL,55539 Point HL at the screen buffer, ScreenBuffer.
54642 LD DE,22496 Point DE at the last line of the bitmap of the screen.
54645 LD BC,8384 Set the width to 32 bytes and the number of lines to 192.
UpdateScreen_Line 54648 PUSH BC Stash the counters on the stack.
54649 PUSH DE Stash the address of the line on the screen on the stack.
54650 PUSH HL Stash the address of the line in the buffer on the stack.
UpdateScreen_Byte 54651 LD A,(HL) Fetch a byte from the buffer.
54652 LD (DE),A Copy it to the screen.
54653 LD (HL),0 Clear the byte in the buffer.
54655 INC HL Move on in the buffer.
54656 INC E Move on in the screen.
54657 DJNZ UpdateScreen_Byte Jump to UpdateScreen_Byte until the line is copied.
54659 POP HL Restore the address of the line in the buffer from the stack.
54660 LD BC,32 Set BC to the length of a line.
54663 ADD HL,BC Move on to the next line in the buffer.
54664 POP DE Restore the address of the line on the screen from the stack.
54665 DEC D Go up one line on the screen.
54666 LD A,D Test whether that has gone into the character row above.
54667 CPL
54668 AND %00000111 Keep the bits of the line within the character row.
54670 JR NZ,UpdateScreen_Next Jump to UpdateScreen_Next if it has not.
54672 LD A,E Go up one character row.
54673 SUB 32
54675 LD E,A Store it.
54676 JR C,UpdateScreen_Next Jump to UpdateScreen_Next if that has gone into the third of the screen above.
54678 LD A,D Put the line at the bottom of the character row.
54679 ADD A,8
54681 LD D,A Store it.
UpdateScreen_Next 54682 POP BC Restore the counters from the stack.
54683 DEC C Count down the lines.
54684 JR NZ,UpdateScreen_Line Jump to UpdateScreen_Line until all of the lines are copied.
54686 RET Return.
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