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⌂ ∞ Elite on the BBC Micro and NES

Version analysis of ZES1

This code appears in the following versions (click to see it in the source code):

Code variations between these versions are shown below.

       Name: ZES1
       Type: Subroutine
   Category: Utility routines
    Summary: Zero-fill the page whose number is in X



 Arguments:

   X                    The page we want to zero-fill



.ZES1

Code variation 1 of 1Specific to an individual platform

Changes required because the flight version of ZES2 is different, but the function of this routine doesn't change.

Tap on a block to expand it, and tap it again to revert.

Cassette, Docked, 6502SP, Master
Flight
Electron
 LDY #0                 \ If we set Y = SC = 0 and fall through into ZES2
 STY SC                 \ below, then we will zero-fill 255 bytes starting from
                        \ SC - in other words, we will zero-fill the whole of
                        \ page X

 STX SC+1               \ We want to zero-fill page X, so store this in the
                        \ high byte of SC, so SC(1 0) is now pointing to page X

 LDA #0                 \ If we set Y = SC = 0 and fall through into ZES2
 STA SC                 \ below, then we will zero-fill 255 bytes starting from
 TAY                    \ SC, then SC + 255, and then the rest of the page - in
                        \ other words, we will zero-fill the whole of page X

 LDY #0                 \ If we set Y = SC = 0 and fall through into ZES2
 STY SC                 \ below, then we will zero-fill 255 bytes starting from
                        \ SC - in other words, we will zero-fill the whole of
                        \ page X

 STX SC+1               \ We want to zero-fill page X, so store this in the
                        \ high byte of SC, so SC(1 0) is now pointing to page X