#include #include "sn76489.h" ; Stores the samples generated in the previous loop around the ISR. .lcomm sample_left, 1 .lcomm sample_right, 1 ; Number of clock cycles to run the emulated SN76489 for this frame (negative). .lcomm sn76489_clock_step, 2 ; Error in SN76489 clock cycles executed (accumulated across multiple samples). .lcomm sn76489_cycle_error, 2 ; Utility macro to provide labels to access the four SN76489 channels. .macro sn76489_define_channel ch .set sn76489_channel_\ch\()_period, (sn76489_channels + \ch * 6 + 0) .set sn76489_channel_\ch\()_amplitude, (sn76489_channels + \ch * 6 + 2) .set sn76489_channel_\ch\()_counter, (sn76489_channels + \ch * 6 + 3) .set sn76489_channel_\ch\()_state, (sn76489_channels + \ch * 6 + 5) .endm ; Define labels to access the four SN76489 channels. sn76489_define_channel 0 sn76489_define_channel 1 sn76489_define_channel 2 sn76489_define_channel 3 ; Updates a square wave tone channel. .macro sn76489_update_tone_channel ch ; Decrement the tone counter. lds YL,sn76489_channel_\ch\()_counter+0 lds YH,sn76489_channel_\ch\()_counter+1 lds ZL,sn76489_clock_step+0 lds ZH,sn76489_clock_step+1 add YL,ZL adc YH,ZH ; Has it overflowed? brcs sn76489_channel_\ch\()_not_overflowed ; It has overflowed! lds ZL,sn76489_channel_\ch\()_period+0 lds ZH,sn76489_channel_\ch\()_period+1 add YL,ZL adc YH,ZH ; Invert the state of the channel. lds ZL,sn76489_channel_\ch\()_state com ZL sts sn76489_channel_\ch\()_state,ZL sn76489_channel_\ch\()_not_overflowed: ; Store the counter. sts sn76489_channel_\ch\()_counter+0,YL sts sn76489_channel_\ch\()_counter+1,YH ; Is the channel active? lds ZL,sn76489_channel_\ch\()_state or ZL,ZL breq sn76489_channel_\ch\()_not_active ; The channel is active. lds ZH,sn76489_channel_\ch\()_amplitude lds ZL,sn76489_stereo_mask ; Add the amplitude for the left channel. lds YL,sample_left sbrc ZL,\ch+4 add YL,ZH sts sample_left,YL ; Add the amplitude for the right channel. lds YL,sample_right sbrc ZL,\ch+0 add YL,ZH sts sample_right,YL sn76489_channel_\ch\()_not_active: .endm .global TIMER1_COMPA_vect TIMER1_COMPA_vect: ; Preserve registers before generating samples. in r_sreg_save,_SFR_IO_ADDR(SREG) push ZL push ZH push YL push YH ; Copy the samples generated in the previous loop to the output register. ; This is done at the start of the ISR to ensure that new samples are written at 44100Hz, ; as the emulation takes different amounts of time depending on how many channels are active. lds YL,sample_left sts _SFR_MEM_ADDR(OCR0A),YL lds YL,sample_right sts _SFR_MEM_ADDR(OCR0B),YL ; Generate samples for this moment in time. ; Start by clearing the sample accumulators. clr YL sts sample_left,YL sts sample_right,YL ; Update the three regular tone channels. sn76489_update_tone_channel 0 sn76489_update_tone_channel 1 sn76489_update_tone_channel 2 lds ZH,sn76489_channel_3_amplitude cpi ZH,0 brne sn76489_channel_3_not_muted jmp sn76489_channel_3_not_active sn76489_channel_3_not_muted: ; Update the noise channel. ; Decrement the tone counter. lds YL,sn76489_channel_3_counter+0 lds YH,sn76489_channel_3_counter+1 lds ZL,sn76489_clock_step+0 lds ZH,sn76489_clock_step+1 add YL,ZL adc YH,ZH ; Has it overflowed? brcc sn76489_channel_3_overflowed jmp sn76489_channel_3_not_overflowed ; It has overflowed! sn76489_channel_3_overflowed: ; Update the counter with its period. lds ZL,sn76489_channel_3_period+0 andi ZL,3 breq sn76489_noise_0x10 dec ZL breq sn76489_noise_0x20 dec ZL breq sn76489_noise_0x40 sn76489_noise_c2: lds ZL,sn76489_channel_2_period+0 lds ZH,sn76489_channel_2_period+1 jmp sn76489_noise_updated_counter sn76489_noise_0x40: ldi ZL,0x40 clr ZH jmp sn76489_noise_updated_counter sn76489_noise_0x20: ldi ZL,0x20 clr ZH jmp sn76489_noise_updated_counter sn76489_noise_0x10: ldi ZL,0x10 clr ZH sn76489_noise_updated_counter: ; Add the period to he counter. add YL,ZL adc YH,ZH ; Store the counter value. sts sn76489_channel_3_counter+0,YL sts sn76489_channel_3_counter+1,YH ; Noise channel operates at half the frequency of the others, so check its update flag. lds YL,sn76489_noise_update com YL sts sn76489_noise_update,YL brne sn76489_generate_noise jmp sn76489_channel_3_output_sample sn76489_generate_noise: ; Are we generating periodic noise or white noise? lds YL,sn76489_channel_3_period+0 bst YL,2 brts sn76489_noise_white sn76489_noise_periodic: ; Generating periodic noise. lds YL,sn76489_shift_register_width cpi YL,16 lds YL,sn76489_shift_register+0 lds YH,sn76489_shift_register+1 breq sn76489_noise_periodic_16 sn76489_noise_periodic_15: lsr YH ror YL brcc sn76489_noise_periodic_15_no_carry ori YH,0x40 sn76489_noise_periodic_15_no_carry: ; Store the updated shift register. sts sn76489_shift_register+0,YL sts sn76489_shift_register+1,YH ; We're done. jmp sn76489_channel_3_output_sample sn76489_noise_periodic_16: lsr YH ror YL brcc sn76489_noise_periodic_16_no_carry ori YH,0x80 sn76489_noise_periodic_16_no_carry: ; Store the updated shift register. sts sn76489_shift_register+0,YL sts sn76489_shift_register+1,YH ; We're done. jmp sn76489_channel_3_output_sample sn76489_noise_white: ; Generate white noise. push XL push XH ; "Tap" a single bit when producing white noise. .macro sn76489_tap_bit ; Y contains the tapped bits. lsr YH ror YL brcc sn76489_bit_not_tapped_\@ ; Z contains the shift register. lsr ZH ror ZL ; XL contains the parity. adc XL,XH jmp sn76489_bit_tapped_\@ sn76489_bit_not_tapped_\@\(): lsr ZH ror ZL sn76489_bit_tapped_\@\(): .endm clr XL clr XH lds YL,sn76489_tapped_bits+0 lds YH,sn76489_tapped_bits+1 lds ZL,sn76489_shift_register+0 lds ZH,sn76489_shift_register+1 ; Handle all sixteen possibile tapped bits. sn76489_tap_bit sn76489_tap_bit sn76489_tap_bit sn76489_tap_bit sn76489_tap_bit sn76489_tap_bit sn76489_tap_bit sn76489_tap_bit sn76489_tap_bit sn76489_tap_bit sn76489_tap_bit sn76489_tap_bit sn76489_tap_bit sn76489_tap_bit sn76489_tap_bit sn76489_tap_bit mov ZL,XL pop XH pop XL ; How wide is the shift register? lds YL,sn76489_shift_register_width cpi YL,16 lds YL,sn76489_shift_register+0 lds YH,sn76489_shift_register+1 breq sn76489_noise_white_16 sn76489_noise_white_15: lsr YH ror YL lsr ZL brcc sn76489_noise_white_15_no_carry ori YH,0x40 sn76489_noise_white_15_no_carry: ; Store the updated shift register. sts sn76489_shift_register+0,YL sts sn76489_shift_register+1,YH ; We're done. jmp sn76489_channel_3_output_sample sn76489_noise_white_16: lsr YH ror YL lsr ZL brcc sn76489_noise_white_16_no_carry ori YH,0x80 sn76489_noise_white_16_no_carry: ; Store the updated shift register. sts sn76489_shift_register+0,YL sts sn76489_shift_register+1,YH ; We're done. jmp sn76489_channel_3_output_sample sn76489_channel_3_not_overflowed: sts sn76489_channel_3_counter+0,YL sts sn76489_channel_3_counter+1,YH sn76489_channel_3_output_sample: ; Is the channel active? lds ZL,sn76489_shift_register+0 andi ZL,0x01 brne sn76489_channel_3_not_active ; The channel is active. lds ZH,sn76489_channel_3_amplitude lds ZL,sn76489_stereo_mask ; Add the amplitude for the left channel. lds YL,sample_left sbrc ZL,3+4 add YL,ZH sts sample_left,YL ; Add the amplitude for the right channel. lds YL,sample_right sbrc ZL,3+0 add YL,ZH sts sample_right,YL sn76489_channel_3_not_active: ; If sample_time > 0, decrement it (used for timing). lds YL,vgm_sample_timer+0 lds YH,vgm_sample_timer+1 mov ZL,YL or ZL,YH breq sample_timer_zero ldi ZL,-1 add YL,ZL adc YH,ZL sts vgm_sample_timer+0,YL sts vgm_sample_timer+1,YH sample_timer_zero: ; Calculate the ClockStep ldi YH,-1 sts sn76489_clock_step+1,YH lds YL,sn76489_cycle_error+0 lds YH,sn76489_cycle_error+1 lds ZL,sn76489_frequency+1 lds ZH,sn76489_frequency+2 sub YL,ZL sbc YH,ZH ldi ZL,lo8(2762) ldi ZH,hi8(2762) push ZL clr ZL calculate_clock_step_loop: dec ZL add YL,ZL adc YH,ZH brmi calculate_clock_step_loop sts sn76489_clock_step+0,ZL pop ZL sts sn76489_cycle_error+0,YL sts sn76489_cycle_error+1,YH ; Restore registers. pop YH pop YL pop ZH pop ZL out _SFR_IO_ADDR(SREG),r_sreg_save reti