Class: PSX::SPU

Inherits:
Object
  • Object
show all
Defined in:
lib/psx/spu.rb

Overview

SPU stub — enough to satisfy programs that probe SPUCNT/SPUSTAT and to round-trip data through SPU RAM via FIFO and DMA channel 4.

No audio synthesis. Voice/reverb/etc. registers read back as zero.

Constant Summary collapse

RAM_SIZE = 
512 * 1024
SPU_TRANSFER_ADDR =

IO offsets relative to 0x1F801000

0xDA6
SPU_FIFO = 
0xDA8
SPUCNT = 
0xDAA
SPUDTC = 
0xDAC
SPUSTAT = 
0xDAE
MODE_STOP =

SPUCNT bits 4-5 = transfer mode

0
MODE_MANUAL = 
1
MODE_DMA_W = 
2
MODE_DMA_R = 
3

Instance Method Summary collapse

Constructor Details

#initialize ⇒ SPU

Returns a new instance of SPU.



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# File 'lib/psx/spu.rb', line 24

def initialize
  @ram = ("\x00" * RAM_SIZE).b
  @transfer_addr = 0   # latched value (byte address)
  @current_addr  = 0   # advances during transfers
  @cnt = 0
  @stat = 0
  @dtc = 0
  @fifo = []
end

Instance Method Details

#dma_read_word ⇒ Object

Used by DMA channel 4. Returns one 32-bit word read from SPU RAM at the current transfer pointer, advancing it.



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# File 'lib/psx/spu.rb', line 75

def dma_read_word
  word = 0
  4.times do |i|
    word |= @ram.getbyte(@current_addr) << (i * 8)
    @current_addr = (@current_addr + 1) & (RAM_SIZE - 1)
  end
  word
end

#dma_write_word(word) ⇒ Object 



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# File 'lib/psx/spu.rb', line 84

def dma_write_word(word)
  4.times do |i|
    @ram.setbyte(@current_addr, (word >> (i * 8)) & 0xFF)
    @current_addr = (@current_addr + 1) & (RAM_SIZE - 1)
  end
end

#read16(offset) ⇒ Object 



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# File 'lib/psx/spu.rb', line 34

def read16(offset)
  case offset
  when SPU_TRANSFER_ADDR then @transfer_addr >> 3
  when SPUCNT            then @cnt
  when SPUDTC            then @dtc
  when SPUSTAT           then @stat
  else 0
  end
end

#write16(offset, value) ⇒ Object 



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# File 'lib/psx/spu.rb', line 44

def write16(offset, value)
  v = value & 0xFFFF
  case offset
  when SPU_TRANSFER_ADDR
    @transfer_addr = (v * 8) & (RAM_SIZE - 1)
    @current_addr = @transfer_addr
  when SPU_FIFO
    if mode == MODE_MANUAL
      # In ManualWrite mode each FIFO write streams straight to SPU RAM,
      # advancing the transfer pointer.
      write_word_to_ram(v)
    else
      # In other modes the data is buffered; on the transition to
      # ManualWrite the buffer drains in order.
      @fifo << v
    end
  when SPUCNT
    prev_mode = mode
    @cnt = v
    # SPUSTAT bits 0-5 mirror SPUCNT bits 0-5 (real hardware applies a
    # short delay; we apply immediately, which is enough for software
    # that polls in a loop).
    @stat = (@stat & ~0x3F) | (@cnt & 0x3F)
    drain_fifo if mode == MODE_MANUAL && prev_mode != MODE_MANUAL
  when SPUDTC
    @dtc = v
  end
end