Routines
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45432: Update Screen
Used by the routines at RunGame, DisplayMenu and ShowGameOverScreen.
Copies the whole buffer to the screen. The buffer is held with its bottom line first, so the copy starts with the bottom line of the screen and works its way up.
Set up the addresses of the buffer and of the bottom line of the screen.
UpdateScreen 45432 LD HL,55439 Point HL at the start of the buffer, ScreenBuffer.
45435 LD DE,22496 Point DE at the bottom line of the screen, 22496.
45438 LD C,192 Set a counter in C of 192 lines.
UpdateScreen_Loop 45440 PUSH BC Stash the line counter on the stack.
45441 PUSH DE Stash the screen address on the stack.
45442 LD BC,32 Set a counter in BC of 32 bytes.
45445 LDIR Copy the line to the screen.
45447 POP DE Restore the screen address from the stack.
Move up to the next pixel line of the screen.
45448 DEC D Move up one pixel line.
45449 LD A,D Invert the high byte of the screen address to test its pixel line bits.
45450 CPL
45451 AND %00000111 Keep only the pixel line bits.
45453 JR NZ,UpdateScreen_Next Jump to UpdateScreen_Next if the line is still in the same character row.
45455 LD A,E Move up one character row.
45456 SUB 32
45458 LD E,A
45459 JR C,UpdateScreen_Next Jump to UpdateScreen_Next if that has crossed into the third above.
45461 LD A,D Otherwise cancel the effect of the borrow on the third, by adding 8 to the high byte.
45462 ADD A,8
45464 LD D,A
UpdateScreen_Next 45465 POP BC Restore the line counter from the stack.
45466 DEC C Decrease the line counter.
45467 JR NZ,UpdateScreen_Loop Jump to UpdateScreen_Loop until all the lines have been copied.
45469 RET Return.
View the equivalent code in;
Before After UpdateScreen
update-screen-before update-screen-after
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