## ASGB Assembler

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    ACKNOWLEDGEMENT

       Thanks  to  Roger Ivie for his contribution of the ASGB cross
    assembler.

            Roger Ivie
            ivie at cc dot usu dot edu

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    INTRODUCTION

       The  Gameboy uses an 8-bit processor which is closely related
    to the 8080.  It is usually described as a modified Z80, but may
    be more closely understood as an enhanced 8080;  it has the 8080
    register set and many, but not all, enhanced  Z80  instructions.
    However,  even  this is not accurate, for the Gameboy also lacks
    some basic 8080 instructions (most annoyingly  SHLD  and  LHLD).
    ASGB is based on ASZ80 and therefore uses the Z80 mnemonic set.

    GAMEBOY REGISTER SET AND CONDITIONS

       The following is a complete list of register designations and
    condition mnemonics:

            byte registers - a,b,c,d,e,h,l
            register pairs - af, bc, de, hl
            word registers - pc, sp

            C  - carry bit set
            NC - carry bit clear
            NZ - zero bit clear
            Z  - zero bit set

    GAMEBOY INSTRUCTION SET

       The following tables list all Gameboy mnemnoics recognized by
    the ASGB assembler.  The designation [] refers to a required ad-
    dressing mode argument.  The following list specifies the format
    for each addressing mode supported by ASGB:

            #data           immediate data
                            byte or word data

            n               byte value

            rg              a byte register
                            a,b,c,d,e,h,l

            rp              a register pair or 16-bit register
                            bc,de,hl

            (hl)            implied addressing or
                            register indirect addressing

            (label)         direct addressing

            label           call/jmp/jr label

       The terms data, dir, and ext may all be expression.  The term
    dir is not allowed to be an external reference.

       Note  that  not all addressing modes are valid with every in-
    struction.  Although official information is not, as  far  as  I
    know, publically available for the Gameboy processor, many unof-
    ficial sources are available on the internet.

    .tile Directive

    Format: .tile /string/

    where:  string  is  a  string of ascii characters taken from the
                    set ' ', '.', '+', '*', '0', '1', '2', and  '3'.
                    The   string   must   be  a  multiple  of  eight
                    characters long.

            /  /     represent  the  delimiting  characters.   These
                    delimiters   may   be   any   paired    printing
                    characters,  as  long  as the characters are not
                    contained within  the  string  itself.   If  the
                    delimiting  characters  do  not match, the .tile
                    directive will give the <q> error.

         The Gameboy displays information on the screen using a pro-
    grammable character set (referred to as  "tiles"  among  Gameboy
    developers).   The ASGB cross assembler has a processor-specific
    assembler directive  to  aid  in  the  creation  of  the  game's
    character set.

         Each  character is created from an 8x8 grid of pixels, each
    pixel of which is composed of two bits.  The .tile directive ac-
    cepts  a single string argument which is processed to create the
    byte  values  corresponding  to  the  lines  of  pixels  in  the
    character.   The  string  argument  must  be  some multiple of 8
    characters long, and be one of these characters:

            ' ' or '0' - for the pixel value 00
            '.' or '1' - for the pixel value 01
            '+' or '2' - for the pixel value 10
            '*' or '3' - for the pixel value 11

         The .tile directive processes each 8-character group of its
    string argument to create the two-byte  value  corresponding  to
    that  line of pixels.  The example in the popular extant litera-
    ture could be done using ASGB like this:

       0000 7C 7C                 1         .tile " *****  "
       0002 00 C6                 2         .tile "++   ++ "
       0004 C6 00                 3         .tile "..   .. "
       0006 00 FE                 4         .tile "+++++++ "
       0008 C6 C6                 5         .tile "**   ** "
       000A 00 C6                 6         .tile "++   ++ "
       000C C6 00                 7         .tile "..   .. "
       000E 00 00                 8         .tile "        "

         Or, using the synonym character set, as:

       0010 7C 7C                10         .tile "03333300"
       0012 00 C6                11         .tile "22000220"
       0014 C6 00                12         .tile "11000110"
       0016 00 FE                13         .tile "22222220"
       0018 C6 C6                14         .tile "33000330"
       001A 00 C6                15         .tile "22000220"
       001C C6 00                16         .tile "11000110"
       001E 00 00                17         .tile "00000000"

         Since .tile is perfectly willing to assemble multiple lines
    of a character at once (as long as it is given complete rows  of
    pixels), it could even be done as:

            .tile " *****  ++   ++ ..   .. +++++++ "
            .tile "**   ** ++   ++ ..   ..         "

    Potentially Controversial Mnemonic Selection

         Although the Gameboy processor is based on the Z80, it does
    include some features which are not present in the Z80.  The Z80
    mnemonic  set  is  not  sufficient  to describe these additional
    operations;  mnemonics must be created for the  new  operations.
    The  mnemonics ASGB uses are not the same as those used by other
    publically-available Gameboy assemblers.

    Auto-Indexing Loads  -

         The  Gameboy provides instructions to load or store the ac-
    cumulator indirectly via HL and then subsequently  increment  or
    decrement HL.  ASGB uses the mnemonic 'ldd' for the instructions
    which decrement HL and 'ldi' for the instructions  which  incre-
    ment  HL.   Because the Gameboy lacks the Z80's block moves, the
    mnemonics are not otherwise needed by ASGB.

            ldd a,(hl)      ldd (hl),a
            ldi a,(hl)      ldi (hl),a

    Input and Output Operations  -

         The  Gameboy  replaces the Z80's separate address space for
    I/O with a mechanism similar to the zero page addressing of pro-
    cessors  such  as  the  6800  or 6502.  All I/O registers in the
    Gameboy reside in the address range between 0xff00  and  0xffff.
    The  Gameboy adds special instructions to load and store the ac-
    cumulator from and into this page of memory.   The  instructions
    are  analogous to the Z80's in and out instructions and ASGB re-
    tains the 'in' and 'out' mnemonics for them.

            in a,(n)        out (n),a
            in a,(c)        out (c),a

         From  ASGB's  perspective,  the  RAM  available from 0xff80
    through 0xffff is composed of unused I/O locations  rather  than
    direct-page RAM.

    The 'stop' Instruction  -

         The  publically-available  documentation  for  the  Gameboy
    lists the 'stop' instruction as the two-byte instruction 10  00,
    and the other freely-available Gameboy assemblers assemble it in
    that manner.  As far as I can tell, the only rationale for  this
    is  that  the  corresponding Z80 instruction ('djnz label') is a
    two-byte instruction.  ASGB assembles 'stop' as the one-byte in-
    struction 10.

    Inherent Instructions

            ccf             cpl
            daa             di
            ei              nop
            halt            rla
            rlca            rra
            rrca            scf
            reti            stop
            swap

    Implicit Operand Instructions

            adc a,[]        adc []
            add a,[]        add []
            and a,[]        and []
            cp a,[]         cp []
            dec a,[]        dec []
            inc a,[]        inc []
            or a,[]         or []
            rl a,[]         rl []
            rlc a,[]        rlc []
            rr a,[]         rr []
            rrc a,[]        rrc []
            sbc a,[]        sbc []
            sla a,[]        sla []
            sra a,[]        sra []
            srl a,[]        srl []
            sub a,[]        sub []
            xor a,[]        xor []

    Load Instructions

            ld rg,[]        ld [],rg
            ld (bc),a       ld a,(bc)
            ld (de),a       ld a,(de)
            ld (label),a    ld a,(label)
            ld (label),sp   ld rp,#data
            ld sp,hl        ld hl,sp

            ldd a,(hl)      ldd (hl),a
            ldi a,(hl)      ldi (hl),a

    Call/Return Instructions

            call C,label    ret C
            call NC,label   ret NC
            call Z,label    ret Z
            call NZ,label   ret NZ
            call label      ret

            rst n

    Jump Instructions

            jp C,label      jp NC,label
            jp Z,label      jp NZ,label

            jp (hl)         jp label

            jr C,label      jr NC,label
            jr Z,label      jr NZ,label
            jr label

    Bit Manipulation Instructions

            bit n,[]
            res n,[]
            set n,[]

    Input and Output Instructions

            in a,(n)        in a,(c)
            out (n),a       out (c),a

    Register Pair Instructions

            add hl,rp       add hl,sp
            add sp,#data

            push rp         pop rp

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Last Updated: March 2021