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⌂ ∞ ▽ The Sentinel on the BBC Micro

Setup: ClearMemory

       Name: ClearMemory                                             [Show more]
       Type: Subroutine
   Category: Setup
    Summary: Clear game memory, so that the BREAK key can remove all trace of
             the game code in early versions of the operating system

    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * ConfigureMachine calls ClearMemory




.ClearMemory

 LDA #&04               \ Set (Q P) = &0400
 STA Q                  \
 LDA #&00               \ We use this as the start address for clearing memory,
 STA P                  \ which is where the game code starts
                        \
                        \ This also sets A = 0, which we can use to zero memory

.cmem1

 LDY #&FF               \ Set Y = &FF to use as a byte counter

.cmem2

 STA (P),Y              \ Zero the Y-th byte at (Q P)

 DEY                    \ Decrement the byte counter

 BNE cmem2              \ Loop back until we have zeroed a whole page of memory

 INC Q                  \ Increment the high byte of (Q P) to point to the next
                        \ page in memory

 LDX Q                  \ Loop back until (Q P) reaches &7C00, at which point we
 CPX #&7C               \ have zeroed all the game code (including any code
 BCC cmem1              \ still left at the higher memory address where it's
                        \ first loaded)

 LDA #&00               \ Set the Break Intercept code to a BRK instruction to
 STA BRKIV              \ reinstate the default break handler

 RTS                    \ Return from the subroutine

 EQUB &20, &65, &74, &73, &36, &0D, &12, &CA    \ These bytes are unused until
 EQUB &19, &20, &20, &20, &20, &20, &20, &4C    \ the game is in progress, at
 EQUB &44, &58, &23, &36, &3A, &4A, &53, &52    \ which point this whole section
 EQUB &20, &43, &46, &4C, &53, &48, &0D, &12    \ of memory is reused
 EQUB &D4, &05, &20, &0D, &12, &DE, &0D, &2E    \
 EQUB &65, &74, &73, &36, &20, &72, &74, &73    \ These values are workspace
 EQUB &0D, &12, &E8, &05, &20, &0D, &12, &F2    \ noise and have no meaning
 EQUB &05, &20, &0D, &12, &FC, &05, &20, &0D    \
 EQUB &13, &06, &05, &20, &0D, &13, &10, &05    \ They actually contain snippets
 EQUB &20, &0D, &13, &1A, &05, &20, &0D, &13    \ of the original source code
 EQUB &24, &05, &20, &0D, &13, &2E, &2A, &2E
 EQUB &4D, &49, &4E, &49, &20, &4C, &44, &41
 EQUB &23, &31, &32, &38, &3A, &53, &54, &41
 EQUB &20, &4D, &45, &41, &4E, &59, &2C, &58
 EQUB &3A, &53, &54, &41, &20, &4D, &45, &4D
 EQUB &4F, &52, &59, &2C, &58, &0D, &13, &38
 EQUB &1F, &20, &20, &20, &20, &20, &20, &4C
 EQUB &44, &41, &23, &30, &3A, &53, &54, &41
 EQUB &20, &4D, &45, &41, &4E, &59, &53, &43
 EQUB &41, &4E, &2C, &58, &0D, &13, &42, &22
 EQUB &20, &20, &20, &20, &20, &20, &4C, &44
 EQUB &41, &23, &36, &34, &3A, &53, &54, &41
 EQUB &20, &4D, &54, &52, &59, &43, &4E, &54
 EQUB &2C, &58, &3A, &72, &74, &73, &0D, &13
 EQUB &4C, &05, &20, &0D, &13, &56, &1A, &2E
 EQUB &4D, &45, &41, &4E, &20, &4C, &44, &41
 EQUB &23, &34, &30, &3A, &53, &54, &41, &20
 EQUB &43, &4F, &56, &45, &52, &0D, &13, &60
 EQUB &1B, &20, &20, &20, &20, &20, &20, &4C
 EQUB &44, &58, &20, &45, &54, &45, &4D, &3A
 EQUB &53, &54, &58, &20, &58, &54