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Elite on the BBC Micro

Big Code File source [Cassette version]

ELITE BIG CODE FILE SOURCE Elite was written by Ian Bell and David Braben and is copyright Acornsoft 1984 The code on this site is identical to the source discs released on Ian Bell's personal website at http://www.elitehomepage.org/ (it's just been reformatted to be more readable) The commentary is copyright Mark Moxon, and any misunderstandings or mistakes in the documentation are entirely my fault The terminology and notations used in this commentary are explained at https://www.bbcelite.com/about_site/terminology_used_in_this_commentary.html The deep dive articles referred to in this commentary can be found at https://www.bbcelite.com/deep_dives
This source file produces the following binary files: * ELTcode.unprot.bin * ELThead.bin after reading in the following files: * ELTA.bin * ELTB.bin * ELTC.bin * ELTD.bin * ELTE.bin * ELTF.bin * ELTG.bin * SHIPS.bin
INCLUDE "1-source-files/main-sources/elite-header.h.asm" _SOURCE_DISC = (_RELEASE = 1) _TEXT_SOURCES = (_RELEASE = 2) GUARD &8000 \ Guard against assembling over MOS memory
Configuration variables
CODE% = &0F40 \ CODE% is set to the location that the main game code \ gets moved to after it is loaded LOAD% = &1128 \ LOAD% points to the start of the actual game code, \ after the &28 bytes of header code that are inserted \ below
Name: ZP [Show more] Type: Workspace Address: &0070 to &0071 Category: Workspaces Summary: Important variables used by the loader
Context: See this workspace on its own page Variations: See code variations for this workspace in the different versions References: This workspace is used as follows: * LBL calls ZP
ORG &0070 .ZP SKIP 2 \ Stores addresses used for moving content around
Load the compiled binaries to create the Big Code File
ORG &1100 \ The load address of the main game code file ("ELTcode" \ for loading from disc, "ELITEcode" for loading from \ tape)
Name: LBL [Show more] Type: Subroutine Category: Copy protection Summary: Checksum the two pages of code that were copied from UU% to LE%
Context: See this subroutine on its own page References: No direct references to this subroutine in this source file

This routine is called at LBL+1 from the CHECKER routine in the loader code in elite-loader.asm. It calculates the checksum of the first two pages of the loader code that was copied from UU% to LE% by part 3 of the loader, and checks the result against the result in the first byte of the LE% block, CHECKbyt, at address &0B00. Other entry points: LBL+2 Contains an RTS
.LBL EQUB &6C \ This value is decremented by the tape loading routine \ in the loader, in IRQ1. During loading this value gets \ decremented down to &6C, and this new value is then \ included in the checksum calculation for the MAINSUM \ checksum in the CHECKER routine (the value is set to \ &6C here as the tape protection is disabled) LDX #&60 \ Set X = &60. This value of X isn't used, it's just a \ set up for the RTS call below, where we jump to LBL+2 \ to perform an RTS, as the opcode for RTS is &60 \ We now run a checksum on the block of memory from \ &0B01 to &0CFF, which is the UU% routine from the \ loader LDA #&B \ Set ZP(1 0) = &0B00, to point to the start of the code STA ZP+1 \ we want to checksum LDY #0 \ Set Y = 0 to count through each byte within each page STY ZP TYA \ Set A = 0 for building the checksum INY \ Increment Y to 1 .CHK3 CLC \ Add the Y-th byte of the game code to A ADC (ZP),Y INY \ Increment the counter to point to the next byte BNE CHK3 \ Loop back for the next byte until we have finished \ adding up this page INC ZP+1 \ Increment the high byte of ZP(1 0) to point to the \ next page .CHK4 CLC \ Add the Y-th byte of this page to the checksum in A ADC (ZP),Y INY \ Increment the counter for this page BPL CHK4 \ Loop back for the next byte until we have finished \ adding up this second page CMP &0B00 \ Compare the result to the contents of CHECKbyt in the \ loader code at elite-loader.asm. This values gets set \ by elite-checksum.py BEQ LBL+2 \ If the checksums match, jump to LBL+2, which contains \ an RTS \ Otherwise the checksum just failed, so we reset the \ machine LDA #%01111111 \ Set 6522 System VIA interrupt enable register IER STA &FE4E \ (SHEILA &4E) bits 0-6 (i.e. disable all hardware \ interrupts from the System VIA) JMP (&FFFC) \ Jump to the address in &FFFC to reset the machine .elitea PRINT "elitea = ", ~P% INCBIN "3-assembled-output/ELTA.bin" .eliteb PRINT "eliteb = ", ~P% INCBIN "3-assembled-output/ELTB.bin" .elitec PRINT "elitec = ", ~P% INCBIN "3-assembled-output/ELTC.bin" .elited PRINT "elited = ", ~P% INCBIN "3-assembled-output/ELTD.bin" .elitee PRINT "elitee = ", ~P% INCBIN "3-assembled-output/ELTE.bin" .elitef PRINT "elitef = ", ~P% INCBIN "3-assembled-output/ELTF.bin" .eliteg PRINT "eliteg = ", ~P% INCBIN "3-assembled-output/ELTG.bin" .checksum0 PRINT "checksum0 = ", ~P% SKIP 1 \ We skip this byte so we can insert the checksum later \ in elite-checksum.py .ships PRINT "ships = ", ~P% INCBIN "3-assembled-output/SHIPS.bin" .end
Save ELTcode.unprot.bin and ELThead.bin
PRINT "P% = ", ~P% PRINT "S.ELTcode 1100 ", ~(LOAD% + &6000 - CODE%), " ", ~LOAD%, ~LOAD% SAVE "3-assembled-output/ELTcode.unprot.bin", &1100, (LOAD% + &6000 - CODE%), LOAD% SAVE "3-assembled-output/ELThead.bin", &1100, elitea, &1100