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elite-source-bank-7.asm
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; ******************************************************************************
;
; NES ELITE GAME SOURCE (BANK 7)
;
; NES Elite was written by Ian Bell and David Braben and is copyright D. Braben
; and I. Bell 1991/1992
;
; The code in this file has been reconstructed from a disassembly of the version
; released on Ian Bell's personal website at http://www.elitehomepage.org/
;
; 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://elite.bbcelite.com/terminology
;
; The deep dive articles referred to in this commentary can be found at
; https://elite.bbcelite.com/deep_dives
;
; ------------------------------------------------------------------------------
;
; This source file contains the game code for ROM bank 7 of NES Elite.
;
; ------------------------------------------------------------------------------
;
; This source file produces the following binary file:
;
; * bank7.bin
;
; ******************************************************************************
; ******************************************************************************
;
; ELITE BANK 7
;
; Produces the binary file bank7.bin.
;
; ******************************************************************************
CODE_BANK_7% = $C000 ; The address where the code will be run
LOAD_BANK_7% = $C000 ; The address where the code will be loaded
ORG CODE_BANK_7%
; ******************************************************************************
;
; Name: ResetMMC1_b7
; Type: Variable
; Category: Start and end
; Summary: The MMC1 mapper reset routine at the start of the ROM bank
; Deep dive: Splitting NES Elite across multiple ROM banks
;
; ------------------------------------------------------------------------------
;
; When the NES is switched on, it is hardwired to perform a JMP ($FFFC). At this
; point, there is no guarantee as to which ROM banks are mapped to $8000 and
; $C000, so to ensure that the game starts up correctly, we put the same code
; in each ROM at the following locations:
;
; * We put $C000 in address $FFFC in every ROM bank, so the NES always jumps
; to $C000 when it starts up via the JMP ($FFFC), irrespective of which
; ROM bank is mapped to $C000.
;
; * We put the same reset routine (this routine, ResetMMC1) at the start of
; every ROM bank, so the same routine gets run, whichever ROM bank is mapped
; to $C000.
;
; This ResetMMC1 routine is therefore called when the NES starts up, whatever
; bank configuration ends up being. It then switches ROM bank 7 to $C000 and
; jumps into bank 7 at the game's entry point BEGIN, which starts the game.
;
; We need to give a different label to this version of the reset routine so we
; can assemble bank 7 at the same time as banks 0 to 6, to enable the lower
; banks to see the exported addresses for bank 7.
;
; ******************************************************************************
.ResetMMC1_b7
SEI ; Disable interrupts
INC $C006 ; Reset the MMC1 mapper, which we can do by writing a
; value with bit 7 set into any address in ROM space
; (i.e. any address from $8000 to $FFFF)
;
; The INC instruction does this in a more efficient
; manner than an LDA/STA pair, as it:
;
; * Fetches the contents of address $C006, which
; contains the high byte of the JMP destination
; below, i.e. the high byte of BEGIN, which is $C0
;
; * Adds 1, to give $C1
;
; * Writes the value $C1 back to address $C006
;
; $C006 is in the ROM space and $C1 has bit 7 set, so
; the INC does all that is required to reset the mapper,
; in fewer cycles and bytes than an LDA/STA pair
;
; Resetting MMC1 maps bank 7 to $C000 and enables the
; bank at $8000 to be switched, so this instruction
; ensures that bank 7 is present
JMP BEGIN ; Jump to BEGIN in bank 7 to start the game
; ******************************************************************************
;
; Name: BEGIN
; Type: Subroutine
; Category: Start and end
; Summary: Run through the NES initialisation process, reset the variables
; and start the game
;
; ******************************************************************************
.BEGIN
SEI ; Disable interrupts
CLD ; Clear the decimal flag, so we're not in decimal mode
; (this has no effect on the NES, as BCD mode is
; disabled in the NES's CPU, but we do this to ensure
; compatibility with 6502-based debuggers)
LDX #$FF ; Set the stack pointer to $01FF, which is the standard
TXS ; location for the 6502 stack, so this instruction
; effectively resets the stack
LDX #0 ; Set startupDebug = 0 (though this value is never read,
STX startupDebug ; so this has no effect)
LDA #%00010000 ; Configure the PPU by setting PPU_CTRL as follows:
STA PPU_CTRL ;
; * Bits 0-1 = base nametable address %00 ($2000)
; * Bit 2 clear = increment PPU_ADDR by 1 each time
; * Bit 3 clear = sprite pattern table is at $0000
; * Bit 4 set = background pattern table is at $1000
; * Bit 5 clear = sprites are 8x8 pixels
; * Bit 6 clear = use PPU 0 (the only option on a NES)
; * Bit 7 clear = disable VBlank NMI generation
STA ppuCtrlCopy ; Store the new value of PPU_CTRL in ppuCtrlCopy so we
; can check its value without having to access the PPU
LDA #%00000000 ; Configure the PPU by setting PPU_MASK as follows:
STA PPU_MASK ;
; * Bit 0 clear = normal colour (not monochrome)
; * Bit 1 clear = hide leftmost 8 pixels of background
; * Bit 2 clear = hide sprites in leftmost 8 pixels
; * Bit 3 clear = hide background
; * Bit 4 clear = hide sprites
; * Bit 5 clear = do not intensify greens
; * Bit 6 clear = do not intensify blues
; * Bit 7 clear = do not intensify reds
; We now wait for three VBlanks to pass to ensure that
; the PPU has stabilised after starting up
.sper1
LDA PPU_STATUS ; Wait for the first VBlank to pass, which will set bit
BPL sper1 ; 7 of PPU_STATUS (and reading PPU_STATUS clears bit 7,
; ready for the next VBlank)
.sper2
LDA PPU_STATUS ; Wait for the second VBlank to pass
BPL sper2
.sper3
LDA PPU_STATUS ; Wait for the third VBlank to pass
BPL sper3
LDA #0 ; Set K% = 0 (English) to set as the default highlighted
STA K% ; language on the Start screen (see the ChooseLanguage
; routine)
LDA #60 ; Set K%+1 = 60 to use as the value of the third counter
STA K%+1 ; when deciding how long to wait on the Start screen
; before auto-playing the demo (see the ChooseLanguage
; routine)
; Fall through into ResetToStartScreen to reset memory
; and show the Start screen
; ******************************************************************************
;
; Name: ResetToStartScreen
; Type: Subroutine
; Category: Start and end
; Summary: Reset the stack and the game's variables and show the Start screen
;
; ******************************************************************************
.ResetToStartScreen
LDX #$FF ; Set the stack pointer to $01FF, which is the standard
TXS ; location for the 6502 stack, so this instruction
; effectively resets the stack
JSR ResetVariables ; Reset all the RAM (in both the NES and cartridge), as
; it is in an undefined state when the NES is switched
; on, initialise all the game's variables, and switch to
; ROM bank 0
JMP ShowStartScreen ; Jump to ShowStartScreen in bank 0 to show the start
; screen and start the game
; ******************************************************************************
;
; Name: ResetVariables
; Type: Subroutine
; Category: Start and end
; Summary: Reset all the RAM (in both the NES and cartridge), initialise all
; the game's variables, and switch to ROM bank 0
;
; ******************************************************************************
.ResetVariables
LDA #%00000000 ; Configure the PPU by setting PPU_CTRL as follows:
STA PPU_CTRL ;
; * Bits 0-1 = base nametable address %00 ($2000)
; * Bit 2 clear = increment PPU_ADDR by 1 each time
; * Bit 3 clear = sprite pattern table is at $0000
; * Bit 4 clear = background pattern table is at $0000
; * Bit 5 clear = sprites are 8x8 pixels
; * Bit 6 clear = use PPU 0 (the only option on a NES)
; * Bit 7 clear = disable VBlank NMI generation
STA ppuCtrlCopy ; Store the new value of PPU_CTRL in ppuCtrlCopy so we
; can check its value without having to access the PPU
STA PPU_MASK ; Configure the PPU by setting PPU_MASK as follows:
;
; * Bit 0 clear = normal colour (not monochrome)
; * Bit 1 clear = hide leftmost 8 pixels of background
; * Bit 2 clear = hide sprites in leftmost 8 pixels
; * Bit 3 clear = hide background
; * Bit 4 clear = hide sprites
; * Bit 5 clear = do not intensify greens
; * Bit 6 clear = do not intensify blues
; * Bit 7 clear = do not intensify reds
STA setupPPUForIconBar ; Clear bit 7 of setupPPUForIconBar so we do nothing
; when the PPU starts drawing the icon bar
LDA #%01000000 ; Configure the APU Frame Counter as follows:
STA APU_FC ;
; * Bit 6 set = do not trigger an IRQ on the last tick
;
; * Bit 7 clear = select the four-step sequence
INC $C006 ; Reset the MMC1 mapper, which we can do by writing a
; value with bit 7 set into any address in ROM space
; (i.e. any address from $8000 to $FFFF)
;
; The INC instruction does this in a more efficient
; manner than an LDA/STA pair, as it:
;
; * Fetches the contents of address $C006, which
; contains the high byte of the JMP destination
; in the JMP BEGIN instruction, i.e. the high byte
; of BEGIN, which is $C0
;
; * Adds 1, to give $C1
;
; * Writes the value $C1 back to address $C006
;
; $C006 is in the ROM space and $C1 has bit 7 set, so
; the INC does all that is required to reset the mapper,
; in fewer cycles and bytes than an LDA/STA pair
;
; Resetting MMC1 maps bank 7 to $C000 and enables the
; bank at $8000 to be switched, so this instruction
; ensures that bank 7 is present
LDA PPU_STATUS ; Read the PPU_STATUS register, which clears the VBlank
; latch in bit 7, so the following loops will wait for
; three VBlanks in total
.resv1
LDA PPU_STATUS ; Wait for the first VBlank to pass, which will set bit
BPL resv1 ; 7 of PPU_STATUS (and reading PPU_STATUS clears bit 7,
; ready for the next VBlank)
.resv2
LDA PPU_STATUS ; Wait for the second VBlank to pass
BPL resv2
.resv3
LDA PPU_STATUS ; Wait for the third VBlank to pass
BPL resv3
; We now zero the RAM in the NES, as follows:
;
; * Zero page from $0000 to $00FF
;
; * The rest of RAM from $0300 to $05FF
;
; This clears all of the NES's built-in RAM except for
; page 1, which is used for the stack
LDA #0 ; Set A to zero so we can poke it into memory
TAX ; Set X to 0 to use as an index counter as we loop
; through zero page
.resv4
STA ZP,X ; Zero the X-th byte of zero page at ZP
INX ; Increment the byte counter
BNE resv4 ; Loop back until we have zeroed the whole of zero page
; from $0000 to $00FF
LDA #$03 ; Set SC(1 0) = $0300
STA SC+1
LDA #$00
STA SC
TXA ; Set A = 0 once again so we can poke it into memory
LDX #3 ; We now zero three pages of memory at $0300, $0400 and
; $0500, so set a page counter in X
TAY ; Set Y = 0 to use as an index counter for each page of
; memory
.resv5
STA (SC),Y ; Zero the Y-th byte of the page at SC(1 0)
INY ; Increment the byte counter
BNE resv5 ; Loop back until we have zeroed the whole page of
; memory at SC(1 0)
INC SC+1 ; Increment the high byte of SC(1 0) so it points at the
; next page of memory
DEX ; Decrement the page counter
BNE resv5 ; Loop back until we have zeroed three pages of memory
; from $0300 to $05FF
JSR SetupMMC1 ; Configure the MMC1 mapper and page ROM bank 0 into
; memory at $8000
JSR ResetMusic ; Reset the current tune to 0 and stop the music
LDA #%10000000 ; Set A = 0 and set the C flag
ASL A
JSR ResetScreen_b3 ; Reset the screen by clearing down the PPU, setting
; all colours to black, and resetting the screen-related
; variables
JSR SetDrawingPlaneTo0 ; Set the drawing bitplane to 0
JSR ResetBuffers ; Reset the pattern and nametable buffers
LDA #00000000 ; Set DTW6 = %00000000 so lower case is not enabled
STA DTW6
LDA #%11111111 ; Set DTW2 = %11111111 to denote that we are not
STA DTW2 ; currently printing a word
LDA #%11111111 ; Set DTW8 = %11111111 to denote that we do not
STA DTW8 ; capitalise the next character
; Fall through into SetBank0 to page ROM bank 0 into
; memory
; ******************************************************************************
;
; Name: SetBank0
; Type: Subroutine
; Category: Utility routines
; Summary: Page ROM bank 0 into memory at $8000
; Deep dive: Splitting NES Elite across multiple ROM banks
;
; ******************************************************************************
.SetBank0
LDA #0 ; Page ROM bank 0 into memory at $8000 and return from
JMP SetBank ; the subroutine using a tail call
; ******************************************************************************
;
; Name: SetNonZeroBank
; Type: Subroutine
; Category: Utility routines
; Summary: An unused routine that pages a specified ROM bank into memory at
; $8000, but only if it is non-zero
; Deep dive: Splitting NES Elite across multiple ROM banks
;
; ------------------------------------------------------------------------------
;
; Arguments:
;
; A The number of the ROM bank to page into memory at $8000
;
; ******************************************************************************
.SetNonZeroBank
CMP currentBank ; If the ROM bank number in A is non-zero, jump to
BNE SetBank ; SetBank to page bank A into memory, returning from the
; subroutine using a tail call
RTS ; Otherwise return from the subroutine
; ******************************************************************************
;
; Name: ResetBank
; Type: Subroutine
; Category: Utility routines
; Summary: Retrieve a ROM bank number from the stack and page that bank into
; memory at $8000
; Deep dive: Splitting NES Elite across multiple ROM banks
;
; ------------------------------------------------------------------------------
;
; Arguments:
;
; Stack The number of the ROM bank to page into memory at $8000
;
; ******************************************************************************
.ResetBank
PLA ; Retrieve the ROM bank number from the stack into A
; Fall through into SetBank to page ROM bank A into
; memory at $8000
; ******************************************************************************
;
; Name: SetBank
; Type: Subroutine
; Category: Utility routines
; Summary: Page a specified ROM bank into memory at $8000
; Deep dive: Splitting NES Elite across multiple ROM banks
;
; ------------------------------------------------------------------------------
;
; Arguments:
;
; A The number of the ROM bank to page into memory at $8000
;
; ******************************************************************************
.SetBank
DEC runningSetBank ; Decrement runningSetBank from 0 to $FF to denote that
; we are in the process of switching ROM banks
;
; This will disable the call to MakeSounds in the NMI
; handler, which instead will increment runningSetBank
; each time it is called
STA currentBank ; Store the number of the new ROM bank in currentBank
STA $FFFF ; Set the MMC1 PRG bank register (which is mapped to
LSR A ; $C000-$DFFF) to the ROM bank number in A, to map the
STA $FFFF ; specified ROM bank into memory at $8000
LSR A ;
STA $FFFF ; Bit 4 of the ROM bank number will be zero, as A is in
LSR A ; the range 0 to 7, which also ensures that PRG-RAM is
STA $FFFF ; enabled and mapped to $6000-$7FFF
LSR A
STA $FFFF
INC runningSetBank ; Increment runningSetBank again
BNE sban1 ; If runningSetBank is non-zero, then this means the NMI
; handler was called while we were switching the ROM
; bank, in which case MakeSounds won't have been called
; in the NMI handler, so jump to sban1 to call the
; MakeSounds routine now instead
RTS ; Return from the subroutine
.sban1
LDA #0 ; Set runningSetBank = 0 so the NMI handler knows we are
STA runningSetBank ; no longer switching ROM banks
LDA currentBank ; Fetch the number of the ROM bank that is currently
PHA ; paged into memory at $8000 and store it on the stack
TXA ; Store X and Y on the stack
PHA
TYA
PHA
JSR MakeSounds_b6 ; Call the MakeSounds routine to make the current sounds
; (music and sound effects)
PLA ; Retrieve X and Y from the stack
TAY
PLA
TAX
JMP ResetBank ; Fetch the previous ROM bank number from the stack and
; page that bank back into memory at $8000, returning
; from the subroutine using a tail call
; ******************************************************************************
;
; Name: xTitleScreen
; Type: Variable
; Category: Start and end
; Summary: The text column for the title screen's title for each language
; Deep dive: Multi-language support in NES Elite
;
; ******************************************************************************
.xTitleScreen
EQUB 6 ; English
EQUB 6 ; German
EQUB 7 ; French
EQUB 7 ; There is no fourth language, so this byte is ignored
; ******************************************************************************
;
; Name: xSpaceView
; Type: Variable
; Category: Flight
; Summary: The text column for the space view name for each language
; Deep dive: Multi-language support in NES Elite
;
; ******************************************************************************
.xSpaceView
EQUB 11 ; English
EQUB 9 ; German
EQUB 13 ; French
EQUB 10 ; There is no fourth language, so this byte is ignored
IF _NTSC
EQUB $20, $20, $20 ; These bytes appear to be unused
EQUB $20, $10, $00
EQUB $C4, $ED, $5E
EQUB $E5, $22, $E5
EQUB $22, $00, $00
EQUB $ED, $5E, $E5
EQUB $22, $09, $68
EQUB $00, $00, $00
EQUB $00
ELIF _PAL
EQUB $FF, $FF, $FF ; These bytes appear to be unused
EQUB $FF, $FF, $FF
EQUB $FF, $FF, $FF
EQUB $FF, $FF, $FF
EQUB $FF, $FF, $FF
EQUB $FF, $FF, $FF
EQUB $FF, $FF, $FF
EQUB $FF, $FF, $FF
EQUB $FF
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
EQUB $6C ; This byte appears to be unused and just contains
; random workspace noise left over from the assembly
; process
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 HI(INT($2000 * LOG(I%) / LOG(2) + 0.5))
NEXT
ENDIF
; ******************************************************************************
;
; Name: logL
; Type: Variable
; Category: Maths (Arithmetic)
; Summary: Binary logarithm table (low byte)
;
; ------------------------------------------------------------------------------
;
; Byte n contains the low byte of:
;
; 32 * log2(n) * 256
;
; ******************************************************************************
.logL
IF _MATCH_ORIGINAL_BINARIES
EQUB $0D ; This byte appears to be unused and just contains
; random workspace noise left over from the assembly
; process
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 LO(INT($2000 * LOG(I%) / LOG(2) + 0.5))
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
FOR I%, 0, 255
EQUB HI(INT(2^((I% / 2 + 128) / 16) + 0.5))
NEXT
; ******************************************************************************
;
; 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
FOR I%, 0, 255
EQUB HI(INT(2^((I% / 2 + 128.25) / 16) + 0.5))
NEXT
; ******************************************************************************
;
; Name: SNE
; Type: Variable
; Category: Maths (Geometry)
; Summary: Sine/cosine table
; Deep dive: The sine, cosine and arctan tables
; Drawing circles
; Drawing ellipses
;
; ------------------------------------------------------------------------------
;
; This lookup table contains sine values for the first half of a circle, from 0
; to 180 degrees (0 to PI radians). In terms of circle or ellipse line segments,
; there are 64 segments in a circle, so this contains sine values for segments
; 0 to 31.
;
; In terms of segments, to calculate the sine of the angle at segment x, we look
; up the value in SNE + x, and to calculate the cosine of the angle we look up
; the value in SNE + ((x + 16) mod 32).
;
; In terms of radians, to calculate the following:
;
; sin(theta) * 256
;
; where theta is in radians, we look up the value in:
;
; SNE + (theta * 10)
;
; To calculate the following:
;
; cos(theta) * 256
;
; where theta is in radians, look up the value in:
;
; SNE + ((theta * 10) + 16) mod 32
;
; Theta must be between 0 and 3.1 radians, so theta * 10 is between 0 and 31.
;
; ******************************************************************************
.SNE
FOR I%, 0, 31
N = ABS(SIN((I% / 64) * 2 * PI))
IF N >= 1
EQUB 255
ELSE
EQUB INT(256 * N + 0.5)
ENDIF
NEXT
; ******************************************************************************
;
; Name: ACT
; Type: Variable
; Category: Maths (Geometry)
; Summary: Arctan table
; Deep dive: The sine, cosine and arctan tables
;
; ------------------------------------------------------------------------------
;
; This table contains lookup values for arctangent calculations involving angles
; in the range 0 to 45 degrees (or 0 to PI / 4 radians).
;
; To calculate the value of theta in the following:
;
; theta = arctan(t)
;
; where 0 <= t < 1, we look up the value in:
;
; ACT + (t * 32)
;
; The result will be an integer representing the angle in radians, where 256
; represents a full circle of 360 degrees (2 * PI radians). The result of the
; lookup will therefore be an integer in the range 0 to 31, as this represents
; 0 to 45 degrees (0 to PI / 4 radians).
;
; The table does not support values of t >= 1 or t < 0 directly, so if we need
; to calculate the arctangent for an angle greater than 45 degrees, we can apply
; the following calculation to the result from the table:
;
; * For t > 1, arctan(t) = 64 - arctan(1 / t)
;
; For negative values of t where -1 < t < 0, we can apply the following
; calculation to the result from the table:
;
; * For t < 0, arctan(-t) = 128 - arctan(t)
;
; Finally, if t < -1, we can do the first calculation to get arctan(|t|), and
; the second to get arctan(-|t|).
;
; ******************************************************************************
.ACT
FOR I%, 0, 31
EQUB INT((128 / PI) * ATN(I% / 32) + 0.5)
NEXT
; ******************************************************************************
;
; Name: XX21
; Type: Variable
; Category: Drawing ships
; Summary: Ship blueprints lookup table
; Deep dive: Ship blueprints
;
; ******************************************************************************
.XX21
EQUW SHIP_MISSILE ; MSL = 1 = Missile
EQUW SHIP_CORIOLIS ; SST = 2 = Coriolis space station
EQUW SHIP_ESCAPE_POD ; ESC = 3 = Escape pod
EQUW SHIP_PLATE ; PLT = 4 = Alloy plate
EQUW SHIP_CANISTER ; OIL = 5 = Cargo canister
EQUW SHIP_BOULDER ; 6 = Boulder
EQUW SHIP_ASTEROID ; AST = 7 = Asteroid
EQUW SHIP_SPLINTER ; SPL = 8 = Splinter
EQUW SHIP_SHUTTLE ; SHU = 9 = Shuttle
EQUW SHIP_TRANSPORTER ; 10 = Transporter
EQUW SHIP_COBRA_MK_3 ; CYL = 11 = Cobra Mk III
EQUW SHIP_PYTHON ; 12 = Python
EQUW SHIP_BOA ; 13 = Boa
EQUW SHIP_ANACONDA ; ANA = 14 = Anaconda
EQUW SHIP_ROCK_HERMIT ; HER = 15 = Rock hermit (asteroid)
EQUW SHIP_VIPER ; COPS = 16 = Viper
EQUW SHIP_SIDEWINDER ; SH3 = 17 = Sidewinder
EQUW SHIP_MAMBA ; 18 = Mamba
EQUW SHIP_KRAIT ; KRA = 19 = Krait
EQUW SHIP_ADDER ; ADA = 20 = Adder
EQUW SHIP_GECKO ; 21 = Gecko
EQUW SHIP_COBRA_MK_1 ; 22 = Cobra Mk I
EQUW SHIP_WORM ; WRM = 23 = Worm
EQUW SHIP_COBRA_MK_3_P ; CYL2 = 24 = Cobra Mk III (pirate)
EQUW SHIP_ASP_MK_2 ; ASP = 25 = Asp Mk II
EQUW SHIP_PYTHON_P ; 26 = Python (pirate)
EQUW SHIP_FER_DE_LANCE ; 27 = Fer-de-lance
EQUW SHIP_MORAY ; 28 = Moray
EQUW SHIP_THARGOID ; THG = 29 = Thargoid
EQUW SHIP_THARGON ; TGL = 30 = Thargon
EQUW SHIP_CONSTRICTOR ; CON = 31 = Constrictor
EQUW SHIP_COUGAR ; COU = 32 = Cougar
EQUW SHIP_DODO ; DOD = 33 = Dodecahedron ("Dodo") space station
; ******************************************************************************
;
; Name: SendBarNamesToPPU
; Type: Subroutine
; Category: PPU
; Summary: Send the nametable entries for the icon bar to the PPU
; Deep dive: Drawing vector graphics using NES tiles
;
; ------------------------------------------------------------------------------
;
; Nametable data for the icon bar is sent to PPU nametables 0 and 1.
;
; ******************************************************************************
.SendBarNamesToPPU
SUBTRACT_CYCLES 2131 ; Subtract 2131 from the cycle count
LDX iconBarRow ; Set X to the low byte of iconBarRow(1 0), to use in
; the following calculations
STX dataForPPU ; Set dataForPPU(1 0) = nameBuffer0 + iconBarRow(1 0)
LDA iconBarRow+1 ;
CLC ; So dataForPPU(1 0) points to the entry in nametable
ADC #HI(nameBuffer0) ; buffer 0 for the start of the icon bar (the addition
STA dataForPPU+1 ; works because the low byte of nameBuffer0 is 0)
LDA iconBarRow+1 ; Set (A X) = PPU_NAME_0 + iconBarRow(1 0)
ADC #HI(PPU_NAME_0) ;
; The addition works because the low byte of PPU_NAME_0
; is 0
STA PPU_ADDR ; Set PPU_ADDR = (A X)
STX PPU_ADDR ; = PPU_NAME_0 + iconBarRow(1 0)
;
; So PPU_ADDR points to the tile entry in the PPU's
; nametable 0 for the start of the icon bar
LDY #0 ; We now send the nametable entries for the icon bar to
; the PPU's nametable 0, so set a counter in Y
.ibar1
LDA (dataForPPU),Y ; Send the Y-th nametable entry from dataForPPU(1 0) to
STA PPU_DATA ; the PPU
INY ; Increment the loop counter
CPY #2*32 ; Loop back until we have sent 2 rows of 32 tiles
BNE ibar1
LDA iconBarRow+1 ; Set (A X) = PPU_NAME_1 + iconBarRow(1 0)
ADC #HI(PPU_NAME_1-1) ;
; The addition works because the low byte of PPU_NAME_1
; is 0 and because the C flag is set (as we just passed
; through the BNE above)
STA PPU_ADDR ; Set PPU_ADDR = (A X)
STX PPU_ADDR ; = PPU_NAME_1 + iconBarRow(1 0)
;
; So PPU_ADDR points to the tile entry in the PPU's
; nametable 1 for the start of the icon bar
LDY #0 ; We now send the nametable entries for the icon bar to
; the PPU's nametable 1, so set a counter in Y
.ibar2