Instructor Notes

February 27, 2008

Quote

"The prospect of charging money for software was a crime against humanity." (Richard M. Stallman, 1979)

Assignment Reminder

• Hello Sprite: due March 2 midnight

Hello GameBoy and Hello Sprite projects

• also show hello-sprite-white

Review

Sprites

• questions?

Window

• overview: gbspec 640 - 668
• not scrollable, but you can position it with WNDPOSX-7, WNDPOSY
• not transparent. Always on top of background

• gbspec lines 1778 - 1788:

                Lets say WY = 70 and WX = 87.
                The window would be positioned as so:
  
                 0                  80               159
                 ______________________________________
              0 |                                      |
                |                   |                  |
                |                                      |
                |         Background Display           |
                |               Here                   |
                |                                      |
                |                                      |
             70 |         -         +------------------|
                |                   | 80,70            |
                |                   |                  |
                |                   |  Window Display  |
                |                   |       Here       |
                |                   |                  |
                |                   |                  |
            143 |___________________|__________________|
  

• specify Window Data Table with register LCDC (gbspec lines 1525 - 1565)
• turn window off and on using STAT register (gbspec lines 1566 - 1625)
• use LCDC status interrupt with coincidence flag to get HBLANK interrupts

Assembler commands

• help: asm.htm
• MACRO: asm/macro.htm
• SET, PRINT, RSSET, HOME, BSS, .labels (see line 101 in hello-world.asm for example)
• _RS counter
• walkthrough of hello-sprite.inc:

  • LoByteVar sort of like declaring variables in c

  • declaration of a structure
• show docs for rgbds

Hello Sprite Good Timer

• questions?
• how would you time multiple things?

Interrupts

• define Interrupt

• Z80 Interrupt vector not on the GameBoy but there is an IF flag (gbspec line 921 to 964)

• interrupt addresses: gbspec.txt lines 156-180
  • vblank
  • timer (rTMA, rTAC (gbspec lines 1137 - 1155)

  • serial transfer (for connecting multiple GameBoys, for example)

new material

Sprites

• sprite bug (gbspec.txt lines 770-790)
  • use incdecss.inc and the inc/dev macros to prevent this

No$GMB emulator

• breakpoint
• animate, trace

Z80 wrapup-part I

• 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.

below is incomplete notes

• instruction types
  • 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
• addressing modes (page 44 of Z80 user's manual)
  • Immediate: opcode and 1 byte
  • Immediate extended: opcode and 2 byte data
  • Modified Page 0 Addressing: (b5 b4 b3 0 0 0)
  • Relative Addressing
  • Extended Addressing: and 2 byte address

  • Indexed Addressing: not used on GameBoy. on Z80 offers displacement to IX or IY

  • Register Addressing:
  • Implied Addressing: register is implied
  • Register Indirect Addressing: pointer to location in memory
  • Bit Addressing
• 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

• differences between Z80 and GameBoy

  • gbspec lines 60-90:

     The following are added instructions:
    
      ADD  SP,nn             ;nn = signed byte
      LD  (HLI),A            ;Write A to (HL) and increment HL
      LD  (HLD),A            ;Write A to (HL) and decrement HL
      LD  A,(HLI)            ;Write (HL) to A and increment HL
      LD  A,(HLD)            ;Write (HL) to A and decrement HL
      LD  A,($FF00+nn)
      LD  A,($FF00+C)
      LD  ($FF00+nn),A
      LD  ($FF00+C),A
      LD  (nnnn),SP
      LD  HL,SP+nn           ;nn = signed byte
      STOP                   ;Stop processor & screen until button press
      SWAP r                 ;Swap high & low nibbles of r
    
     The following instructions have been removed:
    
      Any command that uses the IX or IY registers.
      All IN/OUT instructions.
      All exchange instructions.
      All commands prefixed by ED (except remapped RETI).
      All conditional jumps/calls/rets on parity/overflow and sign flag.
    
     The following instructions have different opcodes:
    
      LD  A,[nnnn]
      LD  [nnnn],A
      RETI