.COLD ; We start by zeroing three pages of memory from $0400 ; to $05FF, so that zeroes the following: ; ; * The UP from workspace $0400 to $0540 ; ; * The WP from workspace $0580 to $06FB ; ; So this initialises all the variables in the UP and WP ; workspaces LDA #4 ; Set the high byte of SC(1 0) to 4 STA SC+1 LDX #3 ; Set X = 3 to act as a page counter, so we zero three ; whole pages of memory LDA #0 ; Set A = 0 so we can use this to zero memory locations STA SC ; Set the low byte of SC(1 0) to zero, so SC is now set ; to $0400 TAY ; Set Y = 0 to act as a byte counter within each page .zerowksploop STA (SC),Y ; Zero the Y-th byte of SC(1 0) INY ; Increment the byte counter BNE zerowksploop ; Loop back until we have zeroed a whole page of memory INC SC+1 ; Increment the high byte of SC(1 0) to point to the ; next page in memory DEX ; Decrement the page counter in X BNE zerowksploop ; Loop back until we have zeroed all three pages LDA #LO(NMIpissoff) ; Set the NMI interrupt vector in NMIV to point to the STA NMIV ; NMIpissoff routine, which acknowledges NMI interrupts LDA #HI(NMIpissoff) ; and ignores them STA NMIV+1 LDA #LO(CHPR2) ; Set the CHRV interrupt vector in CHRV to point to the STA CHRV ; CHPR2 routine, which prints valid ASCII characters LDA #HI(CHPR2) ; using the CHPR routine (so this replaces the normal STA CHRV+1 ; text-printing routine with Elite's own CHPR routine) LDA #%101 ; Call SETL1 to set the 6510 input/output port to the JSR SETL1 ; following: ; ; * LORAM = 1 ; * HIRAM = 0 ; * CHAREN = 1 ; ; This sets the entire 64K memory map to RAM except for ; the I/O memory map at $D000-$DFFF, which gets mapped ; to registers in the VIC-II video controller chip, the ; SID sound chip, the two CIA I/O chips, and so on ; ; See the memory map at the top of page 264 in the ; Programmer's Reference Guide SEI ; Disable interrupts while we configure the VIC-II, CIA ; and SID chips and update the interrupt handlers IF NOT(USA%) ; We only include this code if USA% is FALSE ; ; It is designed to slow down PAL machines to match the ; speed of NTSC machines (though the GMA86 PAL version ; doesn't actually include this code) ; ; Specifically, it waits until raster line 256 + PALCK ; is reached before continuing LDA #PALCK ; Set A = PALCK, which contains the bottom byte of the ; the raster line that we want to wait for .UKCHK BIT VIC+$11 ; Loop back to UKCHK until bit 7 of VIC-II register $11 BPL UKCHK ; (control register 1) is set ; ; Bit 7 of register $11 contains the top bit of the ; current raster line (which is a 9-bit value), so this ; waits until the raster has reached at least line 256 CMP VIC+$12 ; Loop back to UKCHK until VIC-II register $12 equals BNE UKCHK ; PALCK ; ; VIC-II register $12 contains the bottom byte of the ; current raster line (which is a 9-bit value), and we ; only get here when the top bit of the raster line is ; set, so this waits until we have reached raster line ; 256 + PALCK ENDIF LDA #%00000011 ; Set CIA1 register $0D to enable and disable interrupts STA CIA+$D ; as follows: ; ; * Bit 0 set = configure interrupts generated by ; timer A underflow ; ; * Bit 1 set = configure interrupts generated by ; timer B underflow ; ; * Bits 2-4 clear = do not change configuration of ; other interrupts ; ; * Bit 7 clear = disable interrupts whose ; corresponding bits are set ; ; So this disables interrupts that are generated by ; timer A underflow and timer B underflow, while leaving ; other interrupts as they are STA CIA2+$D ; Set CIA2 register $0D to enable and disable interrupts ; as follows: ; ; * Bit 0 set = configure interrupts generated by ; timer A underflow ; ; * Bit 1 set = configure interrupts generated by ; timer B underflow ; ; * Bits 2-4 clear = do not change configuration of ; other interrupts ; ; * Bit 7 clear = disable interrupts whose ; corresponding bits are set ; ; So this disables interrupts that are generated by ; timer A underflow and timer B underflow, while leaving ; other interrupts as they are ;LDA #2 ; These instructions are commented out in the original ;STA VIC+$20 ; source LDA #%00001111 ; Set SID register $18 to control the sound as follows: STA SID+$18 ; ; * Bits 0-3: set the volume to 15 (maximum) ; ; * Bit 4 clear: disable the low-pass filter ; ; * Bit 5 clear: disable the bandpass filter ; ; * Bit 6 clear: disable the high-pass filter ; ; * Bit 7 clear: enable voice 3 LDX #0 ; Set the raster count to 0 to initialise the raster STX RASTCT ; effects in the COMIRQ handler (such as the split ; screen) INX ; Set bit 0 of VIC register $1A and clear bits 1-3 to STX VIC+$1A ; configure the following interrupts: ; ; * Bit 0 = enable raster interrupt ; ; * Bit 1 = disable sprite-background collision ; interrupt ; ; * Bit 2 = disable sprite-sprite collision interrupt ; ; * Bit 3 = disable light pen interrupt LDA VIC+$11 ; Clear bit 7 of VIC register $11, to clear the top bit AND #%01111111 ; of the raster line that generates the interrupt (as STA VIC+$11 ; the line number is a 9-bit value, with bits 0-7 in VIC ; register $12) LDA #40 ; Set VIC register $11 to 40, so along with bit 7 of VIC STA VIC+$12 ; register $10, this sets the raster interrupt to be ; generated when the raster reaches line 40 LDA l1 ; Set bits 0 to 2 of the 6510 port register at location AND #%11111000 ; l1 to %100 to set the input/output port to the ORA #%00000100 ; following: STA l1 ; ; * LORAM = 0 ; * HIRAM = 0 ; * CHAREN = 1 ; ; and return from the subroutine using a tail call ; ; This sets the entire 64K memory map to RAM ; ; See the memory map at the top of page 265 in the ; Programmer's Reference Guide LDA #%100 ; Set L1M to %100, so the default action of the SETL1 STA L1M ; routine will configure memory as above LDA #LO(NMIpissoff) ; Set the NMI interrupt service hardware vector at $FFFA STA $FFFA ; to point to the NMIpissoff routine, which acknowledges LDA #HI(NMIpissoff) ; NMI interrupts and ignores them STA $FFFB ; ; This ensures that even if the Kernal is not paged into ; memory, NMIs will be processed LDA #HI(COMIRQ1) ; Set the IRQ interrupt service hardware vector at $FFFE STA $FFFF ; to point to COMIRQ1, so it gets called to handle all LDA #LO(COMIRQ1) ; IRQ interrupts and BRK instructions (COMIRQ1 plays the STA $FFFE ; background music and manages the split screen) ; ; This ensures that even if the Kernal is not paged into ; memory, IRQs will be processed CLI ; Re-enable interrupts RTS ; Return from the subroutineName: COLD [Show more] Type: Subroutine Category: Loader Summary: Configure memory, set up interrupt handlers and configure the VIC-II, SID and CIA chipsContext: See this subroutine in context in the source code References: This subroutine is called as follows: * S% calls COLD
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Subroutine CHPR2 (category: Text)
Character print vector handler
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Configuration variable CHRV = $0326
The IBSOUT vector that we intercept with our custom text printing routine
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Configuration variable CIA = $DC00
Registers for the CIA1 I/O interface chip, which are memory-mapped to the 16 bytes from $DC00 to $DC0F (see page 428 of the Programmer's Reference Guide)
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Configuration variable CIA2 = $DD00
Registers for the CIA2 I/O interface chip, which are memory-mapped to the 16 bytes from $DD00 to $DD0F (see page 428 of the Programmer's Reference Guide)
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Subroutine COMIRQ1 (category: Drawing the screen)
The split screen and sound interrupt handler (the IRQ interrupt service hardware vector at $FFFE points here)
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Variable L1M (category: Utility routines)
Temporary storage for the new value of the 6510 input/output port register
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Configuration variable NMIV = $0318
The NMINV vector that we intercept with our custom NMI handler, which just acknowledges NMI interrupts and ignores them
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Subroutine NMIpissoff (category: Loader)
Acknowledge NMI interrupts and ignore them
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Configuration variable PALCK = LO(311)
When USA% is set to FALSE, a timing loop is included in the build that waits until the raster line in PALCK is reached; the LO() extracts the lower byte of the raster line to make it easier to compare with VIC-II register $12, which contains the bottom byte of the 9-bit raster line count
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Variable RASTCT (category: Drawing the screen)
The current raster count, which flips between 0 and 1 on each call to the COMIRQ1 interrupt handler (0 = space view, 1 = dashboard)
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Subroutine SETL1 (category: Utility routines)
Set the 6510 input/output port register to control the memory map
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Configuration variable SID = $D400
Registers for the SID sound synthesis chip, which are memory-mapped to the 29 bytes from $D400 to $D41C (see page 461 of the Programmer's Reference Guide)
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Label UKCHK is local to this routine
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Configuration variable VIC = $D000
Registers for the VIC-II video controller chip, which are memory-mapped to the 46 bytes from $D000 to $D02E (see page 454 of the Programmer's Reference Guide)
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Configuration variable l1 = $0001
The 6510 input/output port register, which we can use to configure the Commodore 64 memory layout (see page 260 of the Programmer's Reference Guide)
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Label zerowksploop is local to this routine