Instructor Notes

March 5, 2008

Assignment Reminders

• Initial Term Project Proposal: due March 9 midnight
• Midterm: March 12
• Hello Noise: March 25th
  • focus on sound effects for the games

Alter gbspec.txt?

• Sound register names

Quote

"Engineering problems are under-defined, there are many solutions, good, bad and indifferent. The art is to arrive at a good solution. This is a creative activity, involving imagination, intuition and deliberate choice." -- Ove Arup

Z80 wrapup: Conclusion

• how does the assembler assemble? From http://www.nersc.gov/vendor_docs/ibm/asm/assembler_passes.htm

  • First Pass
    • Checks to see if the instructions are legal in the current assembly mode.
    • Allocates space for instructions and storage areas you request.
      • Fills in the values of constants, where possible.
    • Builds a symbol table, also called a cross-reference table, and makes an entry in this table for every symbol it encounters in the label field of a statement.
  • Second Pass
    • Examines the operands for symbolic references to storage locations and resolves these symbolic references using information in the symbol table.
    • Ensures that no instructions contain an invalid instruction form.
    • Translates source statements into machine code and constants, thus filling the allocated space with object code.
    • Produces a file containing error messages, if any have occurred.
• timing (T cycles, M cycles) (Z80 manual page 80)
  • 1 T cycle is 1/f of clock
  • each machines cycle is one of
    • 1st is always "fetch" which is 4-6 T cycles long
    • subsequent M cycles move data to or from memory
  • see page 12 of z80 manual
• instruction types (see Z80 manual page vi)
  • 8 bit load group
  • 16 bit load group
  • exchange, block transfer, and search group
  • 8 bit arithmetic group

    • DAA

    • note addressing mode
  • general-purpose arithmetic and CPU control
  • 16 bit arithmetic group

  • rotate and shift group

  • bit set, reset, and test group

  • jump group
  • call and return group
  • input and output group

  == GameBoy Audio: General ==

• 4 sound circuits

• 1 & 2 are PWM, 3 plays wave patterns, and 4 is noise

• 2 sound channels (stereo), called SO1 (left) and SO2 (right)
• There is a line called Vin which can accept input from a microphone. From there you can route it back to either SO1 or SO2
• channels 1,2 and 4 have envelope functions, meaning you can have them automatically get louder or softer over time
• channel 1 has a sweep function, meaning you can get it to sweep up in pitch (chirp) or down automatically
• All channels have a duration parameter, or can be left continuously on

• gbhw-snd.inc is an include file to help your code be more self-documenting when coding sound in your apps

• hello-noise.asm demonstrates some basic concepts for producing sound on the GameBoy

General I/O Registers

• AUDVOL
  • Controls volume of SO1 and SO2 (range 0 through 7, 7 is the loudest)
  • Routes Vin to SO1 and/or SO2
• AUDTERM
  • select which of the 4 circuits goes to which of the 2 output channels
• AUDENA
  • turn all sound on or all sound off
  • read only: find out which sound circuits are on

I/O Registers for Circuits 1 and 2

• AUD1LEN, AUD2LEN
  • set duty cycle and sound length
  • Sound length is 6 bits and = (64-x)*(1/256) seconds

  • Duty cycle. From gbspec lines 1220-1223:

                  Wave Duty: 00: 12.5% ( _--------_--------_-------- )
                             01: 25%   ( __-------__-------__------- )
                             10: 50%   ( ____-----____-----____----- ) (default)
                             11: 75%   ( ______---______---______--- )
    

• AUD1ENV, AUD2ENV
  • set an envelope to get either louder or softer
  • set initial volume of envelope (4 bit)
  • set envelope rate (3 bit). One step = x * (1/64) seconds
• AUD1SWEEP
  • circuit 1 only
  • sweep time (3 bit). Higher is slower
  • sweep up or down
  • change of frequency per shift: x(t) = x(t-1) +- x(t-1)/(2^n)
• AUD1LOW, AUD2LOW
  • lower 8 bits of 11 bit number (x) representing a frequency: freq = 131072/(2048-x) Hz
• AUD1HIGH, AUD2HIGH
  • choose if sound is continuous or indeed only the length you specified in AUD1LEN or AUD2LEN
  • upper 3 bits of 11 bit number (x) (see AUD1LOW, AUD2LOW)
  • trigger sound

Sound Circuit 3

overall

• Plays wave pattern ram at $ff30 to $ff3f
• stores in nibbles, reading high nibble first. 32 nibble memory
• play continuous sounds by continually shutting off circuit and reloading memory

  • tutorial for this

I/O Registers

• AUD3ENA
  • turn circuit 3 on or off
• AUD3LEN
  • 8 bits to set sound length (256-x)*.5 seconds
• AUD3LEVEL
  • bits 5 and 6:
    • 00: mute
    • 01: full volume
    • 02: 1/2 volume
    • 03: 1/4 volume
• AUD3LOW, AUD3HIGH
  • load 11 bit number representing equation for freq
  • trigger sound
  • choose whether or not continuous

Sound Circuit 4

overall

• uses binary polynomials to generate pseudo-random numbers (CRC check)
• mess with the parameters and get some interesting effects

I/O registers

• AUD4LEN --- just like AUD1LEN and AUD2LEN
• AUD4ENV --- just like AUD1ENV and AUD2ENV
• AUD4POLY: Parameters for the pseudo random number generator
  • shift clock frequency (4 bits - )
  • selection of polynomial counter step (7 or 15)
  • dividing ratio of freqs (3 bits)
• AUD4GO
  • set whether or not continuous
  • trigger sound