/
elite-z.asm
9403 lines (7284 loc) · 377 KB
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elite-z.asm
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\ ******************************************************************************
\
\ 6502 SECOND PROCESSOR ELITE GAME SOURCE (I/O PROCESSOR)
\
\ 6502 Second Processor Elite was written by Ian Bell and David Braben and is
\ copyright Acornsoft 1985
\
\ 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/terminology
\
\ 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 file:
\
\ * I.CODE.bin
\
\ ******************************************************************************
INCLUDE "1-source-files/main-sources/elite-build-options.asm"
_SOURCE_DISC = (_VARIANT = 1)
_SNG45 = (_VARIANT = 2)
_EXECUTIVE = (_VARIANT = 3)
GUARD &4000 \ Guard against assembling over screen memory
\ ******************************************************************************
\
\ Configuration variables
\
\ ******************************************************************************
CODE% = &2400 \ The assembly address of the main I/O processor code
LOAD% = &2400 \ The load address of the main I/O processor code
VSCAN = 57 \ Defines the split position in the split-screen mode
Y = 96 \ The centre y-coordinate of the 256 x 192 space view
YELLOW = %00001111 \ Four mode 1 pixels of colour 1 (yellow)
RED = %11110000 \ Four mode 1 pixels of colour 2 (red, magenta or white)
CYAN = %11111111 \ Four mode 1 pixels of colour 3 (cyan or white)
GREEN = %10101111 \ Four mode 1 pixels of colour 3, 1, 3, 1 (cyan/yellow)
WHITE = %11111010 \ Four mode 1 pixels of colour 3, 2, 3, 2 (cyan/red)
MAGENTA = RED \ Four mode 1 pixels of colour 2 (red, magenta or white)
DUST = WHITE \ Four mode 1 pixels of colour 3, 2, 3, 2 (cyan/red)
RED2 = %00000011 \ Two mode 2 pixels of colour 1 (red)
GREEN2 = %00001100 \ Two mode 2 pixels of colour 2 (green)
YELLOW2 = %00001111 \ Two mode 2 pixels of colour 3 (yellow)
BLUE2 = %00110000 \ Two mode 2 pixels of colour 4 (blue)
MAG2 = %00110011 \ Two mode 2 pixels of colour 5 (magenta)
CYAN2 = %00111100 \ Two mode 2 pixels of colour 6 (cyan)
WHITE2 = %00111111 \ Two mode 2 pixels of colour 7 (white)
STRIPE = %00100011 \ Two mode 2 pixels of colour 5, 1 (magenta/red)
PARMAX = 15 \ The number of dashboard parameters transmitted with
\ the #RDPARAMS and OSWRCH 137 <param> commands
IRQ1V = &0204 \ The IRQ1V vector that we intercept to implement the
\ split-screen mode
WRCHV = &020E \ The WRCHV vector that we intercept to implement our
\ own custom OSWRCH commands for communicating over the
\ Tube
WORDV = &020C \ The WORDV vector that we intercept to implement our
\ own custom OSWORD commands for communicating over the
\ Tube
RDCHV = &0210 \ The RDCHV vector that we intercept to add validation
\ when reading characters using OSRDCH
VIA = &FE00 \ Memory-mapped space for accessing internal hardware,
\ such as the video ULA, 6845 CRTC and 6522 VIAs (also
\ known as SHEILA)
NVOSWRCH = &FFCB \ The address for the non-vectored OSWRCH routine
OSWRCH = &FFEE \ The address for the OSWRCH routine
OSBYTE = &FFF4 \ The address for the OSBYTE routine
OSWORD = &FFF1 \ The address for the OSWORD routine
\ ******************************************************************************
\
\ Name: ZP
\ Type: Workspace
\ Address: &0080 to &0089
\ Category: Workspaces
\ Summary: Important variables used by the I/O processor
\
\ ******************************************************************************
\ --- Mod: Code removed for music: -------------------->
\ORG &0080
\ --- And replaced by: -------------------------------->
ORG &0070
\ --- End of replacement ------------------------------>
.ZP
SKIP 0 \ The start of the zero page workspace
.P
SKIP 1 \ Temporary storage, used in a number of places
.Q
SKIP 1 \ Temporary storage, used in a number of places
.R
SKIP 1 \ Temporary storage, used in a number of places
.S
SKIP 1 \ Temporary storage, used in a number of places
.T
SKIP 1 \ Temporary storage, used in a number of places
.SWAP
SKIP 1 \ Temporary storage, used to store a flag that records
\ whether or not we had to swap a line's start and end
\ coordinates around when clipping the line in routine
\ LL145 (the flag is used in places like BLINE to swap
\ them back)
.T1
SKIP 1 \ Temporary storage, used in a number of places
.COL
SKIP 1 \ Temporary storage, used to store colour information
\ when drawing pixels in the dashboard
.OSSC
SKIP 2 \ When the parasite sends an OSWORD command to the I/O
\ processor (i.e. an OSWORD with A = 240 to 255), then
\ the relevant handler routine in the I/O processor is
\ called with OSSC(1 0) pointing to the OSWORD parameter
\ block (i.e. OSSC(1 0) = (Y X) from the original call
\ in the I/O processor)
\ --- Mod: Code added for music: ---------------------->
ORG &0086
.musicWorkspace
SKIP 8 \ Storage for the music player, &0092 to &0099 inclusive
.musicRomNumber
SKIP 1 \ The bank number of the sideways ROM slot containing
\ the music player at &009A
.musicStatus
SKIP 1 \ A flag to determine whether to play the currently
\ selected music:
\
\ * 0 = do not play the music
\
\ * Non-zero = do play the music
\ --- End of added code ------------------------------->
ORG &0090
.XX15
SKIP 0 \ Temporary storage, typically used for storing screen
\ coordinates in line-drawing routines
\
\ There are six bytes of storage, from XX15 TO XX15+5.
\ The first four bytes have the following aliases:
\
\ X1 = XX15
\ Y1 = XX15+1
\ X2 = XX15+2
\ Y2 = XX15+3
\
\ These are typically used for describing lines in terms
\ of screen coordinates, i.e. (X1, Y1) to (X2, Y2)
\
\ The last two bytes of XX15 do not have aliases
.X1
SKIP 1 \ Temporary storage, typically used for x-coordinates in
\ line-drawing routines
.Y1
SKIP 1 \ Temporary storage, typically used for y-coordinates in
\ line-drawing routines
.X2
SKIP 1 \ Temporary storage, typically used for x-coordinates in
\ line-drawing routines
.Y2
SKIP 1 \ Temporary storage, typically used for y-coordinates in
\ line-drawing routines
SKIP 2 \ The last 2 bytes of the XX15 block
.SC
SKIP 1 \ Screen address (low byte)
\
\ Elite draws on-screen by poking bytes directly into
\ screen memory, and SC(1 0) is typically set to the
\ address of the character block containing the pixel
\ we want to draw
.SCH
SKIP 1 \ Screen address (high byte)
\ ******************************************************************************
\
\ Name: TINA
\ Type: Workspace
\ Address: &0B00-&0BFF
\ Category: Workspaces
\ Summary: The code block for the TINA hook
\ Deep dive: The TINA hook
\
\ ------------------------------------------------------------------------------
\
\ To use the TINA hook, this workspace should start with "TINA" and then be
\ followed by code that executes on the I/O processor before the main game code
\ terminates.
\
\ ------------------------------------------------------------------------------
\
\ Other entry points:
\
\ TINA+4 The code to run if the TINA hook is enabled
\
\ ******************************************************************************
ORG &0B00
.TINA
SKIP 4
\ ******************************************************************************
\
\ Name: TABLE
\ Type: Variable
\ Category: Drawing lines
\ Summary: The line buffer for line data transmitted from the parasite
\
\ ------------------------------------------------------------------------------
\
\ Lines are drawn by sending the line coordinates one byte at a time from the
\ parasite, using the OSWRCH 129 and 130 commands. As they are sent, they are
\ stored in the TABLE buffer, until all the points have been received, at which
\ point the line is drawn.
\
\ LINTAB points to the offset of the first free byte within TABLE, so the table
\ can be reset by setting LINTAB to 0.
\
\ ******************************************************************************
ORG &2300
.TABLE
SKIP 256
\ ******************************************************************************
\
\ Name: FONT%
\ Type: Variable
\ Category: Text
\ Summary: A copy of the character definition bitmap table from the MOS ROM
\
\ ------------------------------------------------------------------------------
\
\ This is used by the TT26 routine to save time looking up the character bitmaps
\ from the ROM. Note that FONT% contains just the high byte (i.e. the page
\ number) of the address of this table, rather than the full address.
\
\ The contents of the P.FONT.bin file included here are taken straight from the
\ following three pages in the BBC Micro OS 1.20 ROM:
\
\ ASCII 32-63 are defined in &C000-&C0FF (page 0)
\ ASCII 64-95 are defined in &C100-&C1FF (page 1)
\ ASCII 96-126 are defined in &C200-&C2F0 (page 2)
\
\ The code could look these values up each time (as the cassette version does),
\ but it's quicker to use a lookup table, at the expense of three pages of
\ memory.
\
\ The Executive version uses a different font to the standard OS, which is
\ included in the P.FONTEX.bin file. This means all in-game text uses this new
\ font, which is based on the 1960s Westminster font. The font style is similar
\ to the machine-readable font on cheques, and is in a style that we would now
\ call "retro-futuristic" (though presumably it was just "futuristic" back in
\ 1984).
\
\ ******************************************************************************
ORG CODE%
FONT% = P% DIV 256
IF _SNG45 OR _SOURCE_DISC
INCBIN "1-source-files/fonts/P.FONT.bin"
ELIF _EXECUTIVE
INCBIN "1-source-files/fonts/P.FONTEX.bin"
ENDIF
\ ******************************************************************************
\
\ Name: log
\ Type: Variable
\ Category: Maths (Arithmetic)
\ Summary: Binary logarithm table (high byte)
\
\ ------------------------------------------------------------------------------
\
\ At byte n, the table contains the high byte of:
\
\ &2000 * log10(n) / log10(2) = 32 * 256 * log10(n) / log10(2)
\
\ where log10 is the logarithm to base 10. The change-of-base formula says that:
\
\ log2(n) = log10(n) / log10(2)
\
\ so byte n contains the high byte of:
\
\ 32 * log2(n) * 256
\
\ ******************************************************************************
.log
IF _MATCH_ORIGINAL_BINARIES
IF _SNG45
EQUB &18 \ This byte appears to be unused and just contains
\ random workspace noise left over from the BBC Micro
\ assembly process
ELIF _EXECUTIVE
EQUB &FF \ This byte appears to be unused and just contains
\ random workspace noise left over from the BBC Micro
\ assembly process
ELIF _SOURCE_DISC
EQUB &8E \ This byte appears to be unused and just contains
\ random workspace noise left over from the BBC Micro
\ assembly process
ENDIF
EQUB &00, &20, &32, &40, &4A, &52, &59
EQUB &5F, &65, &6A, &6E, &72, &76, &79, &7D
EQUB &80, &82, &85, &87, &8A, &8C, &8E, &90
EQUB &92, &94, &96, &98, &99, &9B, &9D, &9E
EQUB &A0, &A1, &A2, &A4, &A5, &A6, &A7, &A9
EQUB &AA, &AB, &AC, &AD, &AE, &AF, &B0, &B1
EQUB &B2, &B3, &B4, &B5, &B6, &B7, &B8, &B9
EQUB &B9, &BA, &BB, &BC, &BD, &BD, &BE, &BF
EQUB &BF, &C0, &C1, &C2, &C2, &C3, &C4, &C4
EQUB &C5, &C6, &C6, &C7, &C7, &C8, &C9, &C9
EQUB &CA, &CA, &CB, &CC, &CC, &CD, &CD, &CE
EQUB &CE, &CF, &CF, &D0, &D0, &D1, &D1, &D2
EQUB &D2, &D3, &D3, &D4, &D4, &D5, &D5, &D5
EQUB &D6, &D6, &D7, &D7, &D8, &D8, &D9, &D9
EQUB &D9, &DA, &DA, &DB, &DB, &DB, &DC, &DC
EQUB &DD, &DD, &DD, &DE, &DE, &DE, &DF, &DF
EQUB &E0, &E0, &E0, &E1, &E1, &E1, &E2, &E2
EQUB &E2, &E3, &E3, &E3, &E4, &E4, &E4, &E5
EQUB &E5, &E5, &E6, &E6, &E6, &E7, &E7, &E7
EQUB &E7, &E8, &E8, &E8, &E9, &E9, &E9, &EA
EQUB &EA, &EA, &EA, &EB, &EB, &EB, &EC, &EC
EQUB &EC, &EC, &ED, &ED, &ED, &ED, &EE, &EE
EQUB &EE, &EE, &EF, &EF, &EF, &EF, &F0, &F0
EQUB &F0, &F1, &F1, &F1, &F1, &F1, &F2, &F2
EQUB &F2, &F2, &F3, &F3, &F3, &F3, &F4, &F4
EQUB &F4, &F4, &F5, &F5, &F5, &F5, &F5, &F6
EQUB &F6, &F6, &F6, &F7, &F7, &F7, &F7, &F7
EQUB &F8, &F8, &F8, &F8, &F9, &F9, &F9, &F9
EQUB &F9, &FA, &FA, &FA, &FA, &FA, &FB, &FB
EQUB &FB, &FB, &FB, &FC, &FC, &FC, &FC, &FC
EQUB &FD, &FD, &FD, &FD, &FD, &FD, &FE, &FE
EQUB &FE, &FE, &FE, &FF, &FF, &FF, &FF, &FF
ELSE
SKIP 1
FOR I%, 1, 255
EQUB INT(&2000 * LOG(I%) / LOG(2) + 0.5) DIV 256
NEXT
ENDIF
\ ******************************************************************************
\
\ Name: logL
\ Type: Variable
\ Category: Maths (Arithmetic)
\ Summary: Binary logarithm table (low byte)
\
\ ------------------------------------------------------------------------------
\
\ At byte n, the table contains the high byte of:
\
\ &2000 * log10(n) / log10(2) = 32 * 256 * log10(n) / log10(2)
\
\ where log10 is the logarithm to base 10. The change-of-base formula says that:
\
\ log2(n) = log10(n) / log10(2)
\
\ so byte n contains the low byte of:
\
\ 32 * log2(n) * 256
\
\ ******************************************************************************
.logL
IF _MATCH_ORIGINAL_BINARIES
IF _SNG45
EQUB &86 \ This byte appears to be unused and just contains
\ random workspace noise left over from the BBC Micro
\ assembly process
ELIF _EXECUTIVE
EQUB &FF \ This byte appears to be unused and just contains
\ random workspace noise left over from the BBC Micro
\ assembly process
ELIF _SOURCE_DISC
EQUB &00 \ This byte appears to be unused and just contains
\ random workspace noise left over from the BBC Micro
\ assembly process
ENDIF
EQUB &00, &00, &B8, &00, &4D, &B8, &D5
EQUB &FF, &70, &4D, &B3, &B8, &6A, &D5, &05
EQUB &00, &CC, &70, &EF, &4D, &8D, &B3, &C1
EQUB &B8, &9A, &6A, &28, &D5, &74, &05, &88
EQUB &00, &6B, &CC, &23, &70, &B3, &EF, &22
EQUB &4D, &71, &8D, &A3, &B3, &BD, &C1, &BF
EQUB &B8, &AB, &9A, &84, &6A, &4B, &28, &00
EQUB &D5, &A7, &74, &3E, &05, &C8, &88, &45
EQUB &FF, &B7, &6B, &1D, &CC, &79, &23, &CA
EQUB &70, &13, &B3, &52, &EF, &89, &22, &B8
EQUB &4D, &E0, &71, &00, &8D, &19, &A3, &2C
EQUB &B3, &39, &BD, &3F, &C1, &40, &BF, &3C
EQUB &B8, &32, &AB, &23, &9A, &10, &84, &F7
EQUB &6A, &DB, &4B, &BA, &28, &94, &00, &6B
EQUB &D5, &3E, &A7, &0E, &74, &DA, &3E, &A2
EQUB &05, &67, &C8, &29, &88, &E7, &45, &A3
EQUB &00, &5B, &B7, &11, &6B, &C4, &1D, &75
EQUB &CC, &23, &79, &CE, &23, &77, &CA, &1D
EQUB &70, &C1, &13, &63, &B3, &03, &52, &A1
EQUB &EF, &3C, &89, &D6, &22, &6D, &B8, &03
EQUB &4D, &96, &E0, &28, &71, &B8, &00, &47
EQUB &8D, &D4, &19, &5F, &A3, &E8, &2C, &70
EQUB &B3, &F6, &39, &7B, &BD, &FE, &3F, &80
EQUB &C1, &01, &40, &80, &BF, &FD, &3C, &7A
EQUB &B8, &F5, &32, &6F, &AB, &E7, &23, &5F
EQUB &9A, &D5, &10, &4A, &84, &BE, &F7, &31
EQUB &6A, &A2, &DB, &13, &4B, &82, &BA, &F1
EQUB &28, &5E, &94, &CB, &00, &36, &6B, &A0
EQUB &D5, &0A, &3E, &73, &A7, &DA, &0E, &41
EQUB &74, &A7, &DA, &0C, &3E, &70, &A2, &D3
EQUB &05, &36, &67, &98, &C8, &F8, &29, &59
EQUB &88, &B8, &E7, &16, &45, &74, &A3, &D1
ELSE
SKIP 1
FOR I%, 1, 255
EQUB INT(&2000 * LOG(I%) / LOG(2) + 0.5) MOD 256
NEXT
ENDIF
\ ******************************************************************************
\
\ Name: antilog
\ Type: Variable
\ Category: Maths (Arithmetic)
\ Summary: Binary antilogarithm table
\
\ ------------------------------------------------------------------------------
\
\ At byte n, the table contains:
\
\ 2^((n / 2 + 128) / 16) / 256
\
\ which equals:
\
\ 2^(n / 32 + 8) / 256
\
\ ******************************************************************************
.antilog
IF _MATCH_ORIGINAL_BINARIES
EQUB &01, &01, &01, &01, &01, &01, &01, &01
EQUB &01, &01, &01, &01, &01, &01, &01, &01
EQUB &01, &01, &01, &01, &01, &01, &01, &01
EQUB &01, &01, &01, &01, &01, &01, &01, &01
EQUB &02, &02, &02, &02, &02, &02, &02, &02
EQUB &02, &02, &02, &02, &02, &02, &02, &02
EQUB &02, &02, &02, &03, &03, &03, &03, &03
EQUB &03, &03, &03, &03, &03, &03, &03, &03
EQUB &04, &04, &04, &04, &04, &04, &04, &04
EQUB &04, &04, &04, &05, &05, &05, &05, &05
EQUB &05, &05, &05, &06, &06, &06, &06, &06
EQUB &06, &06, &07, &07, &07, &07, &07, &07
EQUB &08, &08, &08, &08, &08, &08, &09, &09
EQUB &09, &09, &09, &0A, &0A, &0A, &0A, &0B
EQUB &0B, &0B, &0B, &0C, &0C, &0C, &0C, &0D
EQUB &0D, &0D, &0E, &0E, &0E, &0E, &0F, &0F
EQUB &10, &10, &10, &11, &11, &11, &12, &12
EQUB &13, &13, &13, &14, &14, &15, &15, &16
EQUB &16, &17, &17, &18, &18, &19, &19, &1A
EQUB &1A, &1B, &1C, &1C, &1D, &1D, &1E, &1F
EQUB &20, &20, &21, &22, &22, &23, &24, &25
EQUB &26, &26, &27, &28, &29, &2A, &2B, &2C
EQUB &2D, &2E, &2F, &30, &31, &32, &33, &34
EQUB &35, &36, &38, &39, &3A, &3B, &3D, &3E
EQUB &40, &41, &42, &44, &45, &47, &48, &4A
EQUB &4C, &4D, &4F, &51, &52, &54, &56, &58
EQUB &5A, &5C, &5E, &60, &62, &64, &67, &69
EQUB &6B, &6D, &70, &72, &75, &77, &7A, &7D
EQUB &80, &82, &85, &88, &8B, &8E, &91, &94
EQUB &98, &9B, &9E, &A2, &A5, &A9, &AD, &B1
EQUB &B5, &B8, &BD, &C1, &C5, &C9, &CE, &D2
EQUB &D7, &DB, &E0, &E5, &EA, &EF, &F5, &FA
ELSE
FOR I%, 0, 255
EQUB INT(2^((I% / 2 + 128) / 16) + 0.5) DIV 256
NEXT
ENDIF
\ ******************************************************************************
\
\ Name: antilogODD
\ Type: Variable
\ Category: Maths (Arithmetic)
\ Summary: Binary antilogarithm table
\
\ ------------------------------------------------------------------------------
\
\ At byte n, the table contains:
\
\ 2^((n / 2 + 128.25) / 16) / 256
\
\ which equals:
\
\ 2^(n / 32 + 8.015625) / 256 = 2^(n / 32 + 8) * 2^(.015625) / 256
\ = (2^(n / 32 + 8) + 1) / 256
\
\ ******************************************************************************
.antilogODD
IF _MATCH_ORIGINAL_BINARIES
EQUB &01, &01, &01, &01, &01, &01, &01, &01
EQUB &01, &01, &01, &01, &01, &01, &01, &01
EQUB &01, &01, &01, &01, &01, &01, &01, &01
EQUB &01, &01, &01, &01, &01, &01, &01, &01
EQUB &02, &02, &02, &02, &02, &02, &02, &02
EQUB &02, &02, &02, &02, &02, &02, &02, &02
EQUB &02, &02, &02, &03, &03, &03, &03, &03
EQUB &03, &03, &03, &03, &03, &03, &03, &03
EQUB &04, &04, &04, &04, &04, &04, &04, &04
EQUB &04, &04, &05, &05, &05, &05, &05, &05
EQUB &05, &05, &05, &06, &06, &06, &06, &06
EQUB &06, &06, &07, &07, &07, &07, &07, &07
EQUB &08, &08, &08, &08, &08, &09, &09, &09
EQUB &09, &09, &0A, &0A, &0A, &0A, &0A, &0B
EQUB &0B, &0B, &0B, &0C, &0C, &0C, &0D, &0D
EQUB &0D, &0D, &0E, &0E, &0E, &0F, &0F, &0F
EQUB &10, &10, &10, &11, &11, &12, &12, &12
EQUB &13, &13, &14, &14, &14, &15, &15, &16
EQUB &16, &17, &17, &18, &18, &19, &1A, &1A
EQUB &1B, &1B, &1C, &1D, &1D, &1E, &1E, &1F
EQUB &20, &21, &21, &22, &23, &24, &24, &25
EQUB &26, &27, &28, &29, &29, &2A, &2B, &2C
EQUB &2D, &2E, &2F, &30, &31, &32, &34, &35
EQUB &36, &37, &38, &3A, &3B, &3C, &3D, &3F
EQUB &40, &42, &43, &45, &46, &48, &49, &4B
EQUB &4C, &4E, &50, &52, &53, &55, &57, &59
EQUB &5B, &5D, &5F, &61, &63, &65, &68, &6A
EQUB &6C, &6F, &71, &74, &76, &79, &7B, &7E
EQUB &81, &84, &87, &8A, &8D, &90, &93, &96
EQUB &99, &9D, &A0, &A4, &A7, &AB, &AF, &B3
EQUB &B6, &BA, &BF, &C3, &C7, &CB, &D0, &D4
EQUB &D9, &DE, &E3, &E8, &ED, &F2, &F7, &FD
ELSE
FOR I%, 0, 255
EQUB INT(2^((I% / 2 + 128.25) / 16) + 0.5) DIV 256
NEXT
ENDIF
\ ******************************************************************************
\
\ Name: ylookup
\ Type: Variable
\ Category: Drawing pixels
\ Summary: Lookup table for converting pixel y-coordinate to page number of
\ screen address
\
\ ------------------------------------------------------------------------------
\
\ Elite's screen mode is based on mode 1, so it allocates two pages of screen
\ memory to each character row (where a character row is 8 pixels high). This
\ table enables us to convert a pixel y-coordinate in the range 0-247 into the
\ page number for the start of the character row containing that coordinate.
\
\ Screen memory is from &4000 to &7DFF, so the lookup works like this:
\
\ Y = 0 to 7, lookup value = &40 (so row 1 is from &4000 to &41FF)
\ Y = 8 to 15, lookup value = &42 (so row 2 is from &4200 to &43FF)
\ Y = 16 to 23, lookup value = &44 (so row 3 is from &4400 to &45FF)
\ Y = 24 to 31, lookup value = &46 (so row 4 is from &4600 to &47FF)
\
\ ...
\
\ Y = 232 to 239, lookup value = &7A (so row 31 is from &7A00 to &7BFF)
\ Y = 240 to 247, lookup value = &7C (so row 32 is from &7C00 to &7DFF)
\
\ There is also a lookup value for y-coordinates from 248 to 255, but that's off
\ the end of the screen, as the special Elite screen mode only has 31 character
\ rows.
\
\ ******************************************************************************
.ylookup
FOR I%, 0, 255
EQUB &40 + ((I% DIV 8) * 2)
NEXT
\ ******************************************************************************
\
\ Name: TVT3
\ Type: Variable
\ Category: Drawing the screen
\ Summary: Palette data for the mode 1 part of the screen (the top part)
\
\ ------------------------------------------------------------------------------
\
\ The following table contains four different mode 1 palettes, each of which
\ sets a four-colour palette for the top part of the screen. Mode 1 supports
\ four colours on-screen and in Elite colour 0 is always set to black, so each
\ of the palettes in this table defines the three other colours (1 to 3).
\
\ There is some consistency between the palettes:
\
\ * Colour 0 is always black
\ * Colour 1 (#YELLOW) is always yellow
\ * Colour 2 (#RED) is normally red-like (i.e. red or magenta)
\ ... except in the title screen palette, when it is white
\ * Colour 3 (#CYAN) is always cyan-like (i.e. white or cyan)
\
\ The configuration variables of #YELLOW, #RED and #CYAN are a bit misleading,
\ but if you think of them in terms of hue rather than specific colours, they
\ work reasonably well (outside of the title screen palette, anyway).
\
\ The palettes are set in the IRQ1 handler that implements the split screen
\ mode, and can be changed by the parasite sending a #SETVDU19 <offset> command
\ to point to the offset of the new palette in this table.
\
\ This table must start on a page boundary (i.e. an address that ends in two
\ zeroes in hexadecimal). In the release version of the game TVT3 is at &2C00.
\ This is so the #SETVDU19 command can switch palettes properly, as it does this
\ by overwriting the low byte of the palette data address with a new offset, so
\ the low byte for first palette's address must be 0.
\
\ Palette data is given as a set of bytes, with each byte mapping a logical
\ colour to a physical one. In each byte, the logical colour is given in bits
\ 4-7 and the physical colour in bits 0-3. See p.379 of the Advanced User Guide
\ for details of how palette mapping works, as in modes 1 and 2 we have to do
\ multiple palette commands to change the colours correctly, and the physical
\ colour value is EOR'd with 7, just to make things even more confusing.
\
\ ******************************************************************************
.TVT3
EQUB &00, &34 \ 1 = yellow, 2 = red, 3 = cyan (space view)
EQUB &24, &17 \
EQUB &74, &64 \ Set with a #SETVDU19 0 command, after which:
EQUB &57, &47 \
EQUB &B1, &A1 \ #YELLOW = yellow
EQUB &96, &86 \ #RED = red
EQUB &F1, &E1 \ #CYAN = cyan
EQUB &D6, &C6 \ #GREEN = cyan/yellow stripe
\ #WHITE = cyan/red stripe
EQUB &00, &34 \ 1 = yellow, 2 = red, 3 = white (chart view)
EQUB &24, &17 \
EQUB &74, &64 \ Set with a #SETVDU19 16 command, after which:
EQUB &57, &47 \
EQUB &B0, &A0 \ #YELLOW = yellow
EQUB &96, &86 \ #RED = red
EQUB &F0, &E0 \ #CYAN = white
EQUB &D6, &C6 \ #GREEN = white/yellow stripe
\ #WHITE = white/red stripe
EQUB &00, &34 \ 1 = yellow, 2 = white, 3 = cyan (title screen)
EQUB &24, &17 \
EQUB &74, &64 \ Set with a #SETVDU19 32 command, after which:
EQUB &57, &47 \
EQUB &B1, &A1 \ #YELLOW = yellow
EQUB &90, &80 \ #RED = white
EQUB &F1, &E1 \ #CYAN = cyan
EQUB &D0, &C0 \ #GREEN = cyan/yellow stripe
\ #WHITE = cyan/white stripe
EQUB &00, &34 \ 1 = yellow, 2 = magenta, 3 = white (trade view)
EQUB &24, &17 \
EQUB &74, &64 \ Set with a #SETVDU19 48 command, after which:
EQUB &57, &47 \
EQUB &B0, &A0 \ #YELLOW = yellow
EQUB &92, &82 \ #RED = magenta
EQUB &F0, &E0 \ #CYAN = white
EQUB &D2, &C2 \ #GREEN = white/yellow stripe
\ #WHITE = white/magenta stripe
\ ******************************************************************************
\
\ Name: I/O variables
\ Type: Workspace
\ Address: &2C40 to &2C60
\ Category: Workspaces
\ Summary: Various variables used by the I/O processor
\
\ ******************************************************************************
.XC
EQUB 1 \ The x-coordinate of the text cursor (i.e. the text
\ column), set to an initial value of 1
.YC
EQUB 1 \ The y-coordinate of the text cursor (i.e. the text
\ row), set to an initial value of 1
.K3
SKIP 1 \ Temporary storage, used in a number of places
.U
SKIP 1 \ Temporary storage, used in a number of places
.LINTAB
SKIP 1 \ The offset of the first free byte in the TABLE buffer,
\ which stores bytes in the current line as they are
\ transmitted from the parasite using the OSWRCH 129 and
\ 130 commands
.LINMAX
SKIP 1 \ The number of points in the line currently being
\ transmitted from the parasite using the OSWRCH 129
\ and 130 commands
.YSAV
SKIP 1 \ Temporary storage for saving the value of the Y
\ register, used in a number of places
.svn
SKIP 1 \ "Saving in progress" flag
\
\ * Non-zero while the disc is being accessed (so this
\ is also the case for cataloguing, loading etc.)
\
\ * 0 otherwise
.PARANO
SKIP 1 \ PARANO points to the last free byte in PARAMS, which
\ is used as a buffer for bytes sent from the parasite
\ by the #RDPARAMS and OSWRCH 137 <param> commands when
\ updating the dashboard
.DL
SKIP 1 \ Vertical sync flag
\
\ DL gets set to 30 every time we reach vertical sync on
\ the video system, which happens 50 times a second
\ (50Hz). The WSCAN routine uses this to pause until the
\ vertical sync, by setting DL to 0 and then monitoring
\ its value until it changes to 30
.VEC
SKIP 2 \ VEC = &7FFE
\
\ This gets set to the value of the original IRQ1 vector
\ by the loading process
.HFX
SKIP 1 \ A flag that toggles the hyperspace colour effect
\
\ * 0 = no colour effect
\
\ * Non-zero = hyperspace colour effect enabled
\
\ When HFX is set to 1, the mode 1 screen that makes
\ up the top part of the display is temporarily switched
\ to mode 2 (the same screen mode as the dashboard),
\ which has the effect of blurring and colouring the
\ hyperspace rings in the top part of the screen. The
\ code to do this is in the LINSCN routine, which is
\ called as part of the screen mode routine at IRQ1.
\ It's in LINSCN that HFX is checked, and if it is
\ non-zero, the top part of the screen is not switched
\ to mode 1, thus leaving the top part of the screen in
\ the more colourful mode 2
.CATF
SKIP 1 \ The disc catalogue flag
\
\ Determines whether a disc catalogue is currently in
\ progress, so the TT26 print routine can format the
\ output correctly:
\
\ * 0 = disc is not currently being catalogued
\
\ * 1 = disc is currently being catalogued
\
\ Specifically, when CATF is non-zero, TT26 will omit
\ column 17 from the catalogue so that it will fit
\ on-screen (column 17 is blank column in the middle
\ of the catalogue, between the two lists of filenames,
\ so it can be dropped without affecting the layout)
.K
SKIP 4 \ Temporary storage, used in a number of places
.PARAMS
SKIP 0 \ PARAMS points to the start of the dashboard parameter
\ block that is populated by the parasite when it sends
\ the #RDPARAMS and OSWRCH 137 <param> commands
\
\ These commands update the dashboard, but because the
\ parameter block uses the same locations as the flight
\ variables, these commands also have the effect of
\ updating the following variables, from ENERGY to ESCP
.ENERGY
SKIP 1 \ Energy bank status
\
\ * 0 = empty
\
\ * &FF = full
.ALP1
SKIP 1 \ Magnitude of the roll angle alpha, i.e. |alpha|,
\ which is a positive value between 0 and 31
.ALP2
SKIP 1 \ Bit 7 of ALP2 = sign of the roll angle in ALPHA
.BETA
SKIP 1 \ The current pitch angle beta, which is reduced from
\ JSTY to a sign-magnitude value between -8 and +8
\
\ This describes how fast we are pitching our ship, and
\ determines how fast the universe pitches around us
\
\ The sign bit is also stored in BET2, while the
\ opposite sign is stored in BET2+1
.BET1
SKIP 1 \ The magnitude of the pitch angle beta, i.e. |beta|,
\ which is a positive value between 0 and 8
.DELTA
SKIP 1 \ Our current speed, in the range 1-40
.ALTIT
SKIP 1 \ Our altitude above the surface of the planet or sun
\
\ * 255 = we are a long way above the surface
\
\ * 1-254 = our altitude as the square root of:
\
\ x_hi^2 + y_hi^2 + z_hi^2 - 6^2
\
\ where our ship is at the origin, the centre of the
\ planet/sun is at (x_hi, y_hi, z_hi), and the
\ radius of the planet/sun is 6
\
\ * 0 = we have crashed into the surface
.MCNT
SKIP 1 \ The main loop counter
\
\ This counter determines how often certain actions are
\ performed within the main loop. See the deep dive on
\ "Scheduling tasks with the main loop counter" for more
\ details
.FSH
SKIP 1 \ Forward shield status
\
\ * 0 = empty
\