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129 lines
3.9 KiB
NASM
129 lines
3.9 KiB
NASM
// A minimal working raster hardware IRQ with clobber-based register savings
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/// @file
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/// Commodore 64 Registers and Constants
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/// @file
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/// The MOS 6526 Complex Interface Adapter (CIA)
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///
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/// http://archive.6502.org/datasheets/mos_6526_cia_recreated.pdf
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// Commodore 64 PRG executable file
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.file [name="irq-hardware-clobber-jsr.prg", type="prg", segments="Program"]
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.segmentdef Program [segments="Basic, Code, Data"]
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.segmentdef Basic [start=$0801]
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.segmentdef Code [start=$80d]
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.segmentdef Data [startAfter="Code"]
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.segment Basic
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:BasicUpstart(main)
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/// Value that disables all CIA interrupts when stored to the CIA Interrupt registers
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.const CIA_INTERRUPT_CLEAR_ALL = $7f
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/// VICII IRQ Status/Enable Raster
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// @see #IRQ_ENABLE #IRQ_STATUS
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/// 0 | RST| Reaching a certain raster line. The line is specified by writing
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/// | | to register 0xd012 and bit 7 of $d011 and internally stored by
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/// | | the VIC for the raster compare. The test for reaching the
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/// | | interrupt raster line is done in cycle 0 of every line (for line
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/// | | 0, in cycle 1).
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.const IRQ_RASTER = 1
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/// Mask for PROCESSOR_PORT_DDR which allows only memory configuration to be written
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.const PROCPORT_DDR_MEMORY_MASK = 7
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/// RAM in 0xA000, 0xE000 I/O in 0xD000
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.const PROCPORT_RAM_IO = 5
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/// The colors of the C64
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.const BLACK = 0
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.const WHITE = 1
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.const OFFSET_STRUCT_MOS6526_CIA_INTERRUPT = $d
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/// $D012 RASTER Raster counter
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.label RASTER = $d012
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/// $D020 Border Color
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.label BORDER_COLOR = $d020
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/// $D021 Background Color 0
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.label BG_COLOR = $d021
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/// $D011 Control Register #1
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/// - Bit#0-#2: YSCROLL Screen Soft Scroll Vertical
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/// - Bit#3: RSEL Switch betweem 25 or 24 visible rows
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/// RSEL| Display window height | First line | Last line
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/// ----+--------------------------+-------------+----------
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/// 0 | 24 text lines/192 pixels | 55 ($37) | 246 ($f6)
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/// 1 | 25 text lines/200 pixels | 51 ($33) | 250 ($fa)
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/// - Bit#4: DEN Switch VIC-II output on/off
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/// - Bit#5: BMM Turn Bitmap Mode on/off
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/// - Bit#6: ECM Turn Extended Color Mode on/off
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/// - Bit#7: RST8 9th Bit for $D012 Rasterline counter
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/// Initial Value: %10011011
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.label VICII_CONTROL1 = $d011
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/// VIC II IRQ Status Register
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.label IRQ_STATUS = $d019
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/// VIC II IRQ Enable Register
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.label IRQ_ENABLE = $d01a
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/// Processor port data direction register
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.label PROCPORT_DDR = 0
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/// Processor Port Register controlling RAM/ROM configuration and the datasette
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.label PROCPORT = 1
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/// The CIA#1: keyboard matrix, joystick #1/#2
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.label CIA1 = $dc00
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/// The vector used when the HARDWARE serves IRQ interrupts
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.label HARDWARE_IRQ = $fffe
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.segment Code
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// Interrupt Routine
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irq: {
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sta rega+1
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// do_irq()
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jsr do_irq
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// }
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rega:
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lda #0
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rti
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}
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main: {
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// asm
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sei
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// *PROCPORT_DDR = PROCPORT_DDR_MEMORY_MASK
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// Disable kernal & basic
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lda #PROCPORT_DDR_MEMORY_MASK
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sta.z PROCPORT_DDR
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// *PROCPORT = PROCPORT_RAM_IO
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lda #PROCPORT_RAM_IO
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sta.z PROCPORT
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// CIA1->INTERRUPT = CIA_INTERRUPT_CLEAR_ALL
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// Disable CIA 1 Timer IRQ
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lda #CIA_INTERRUPT_CLEAR_ALL
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sta CIA1+OFFSET_STRUCT_MOS6526_CIA_INTERRUPT
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// *VICII_CONTROL1 |=$80
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// Set raster line to $100
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lda #$80
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ora VICII_CONTROL1
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sta VICII_CONTROL1
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// *RASTER = $00
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lda #0
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sta RASTER
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// *IRQ_ENABLE = IRQ_RASTER
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// Enable Raster Interrupt
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lda #IRQ_RASTER
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sta IRQ_ENABLE
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// *HARDWARE_IRQ = &irq
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// Set the IRQ routine
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lda #<irq
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sta HARDWARE_IRQ
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lda #>irq
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sta HARDWARE_IRQ+1
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// asm
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cli
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__b1:
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// (*BORDER_COLOR)++;
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inc BORDER_COLOR
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jmp __b1
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}
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do_irq: {
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// *BG_COLOR = WHITE
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lda #WHITE
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sta BG_COLOR
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// *BG_COLOR = BLACK
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lda #BLACK
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sta BG_COLOR
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// *IRQ_STATUS = IRQ_RASTER
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// Acknowledge the IRQ
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lda #IRQ_RASTER
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sta IRQ_STATUS
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// }
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rts
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}
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