Risc World 3/Risc World 3.iso/SOFTWARE/ISSUE4/ZAP/!Zap/Code/Extensions/ExtAsm/ExtBasDoc

Text File | 2002-05-08 | 22KB | 673 lines

Extended BASIC Assembler
————————————————————————

Version 2.01,         12 June 2000   (BASIC 1.05-1.16)
        2.01 patch 2, 14 August 2000 (BASIC 1.20)
by Darren Salt <[e-mail]>
AOF code by Paul Clifford <[e-mail]>
Contains code from v1.00 and v1.30 by Adrian Lees

(This is a beta version.)


Licence
———————

ExtBASICasm is freeware. No restrictions are placed on its use; what you do
with it is your own responsibility.

Release versions of ExtBASICasm may be freely distributed in unchanged form;
patched versions may not be distributed unless the patches are *clearly*
documented; such documentation must be distributed with the patched module.

Beta versions of ExtBASICasm must not be redistributed.


Intro
—————

It is recommended that you read through this document before first using this
version of ExtBASICasm, and check through it quickly when upgrading. It's
entirely possible that one of the extras the module provides may cause a
clash with existing programs, for example the APCS-R register names clashing
with variables used as register names.

Note also that APCS-R register names are disabled by default.


The module ExtBASICasm provides a patch for various versions of BBC BASIC V
to allow the direct use of the extra instructions provided by the ARM3, ARM6,
ARM7 and StrongARM processors. The missing floating-point and general
coprocessor instructions, and some assembler directives more familiar (and a
few unfamiliar) to Acorn Assembler users have been added; also the APCS-R
register names may be used. Also, AOF files may be generated.

To make the necessary changes to the BASIC module it must be located in RAM.
The ExtBASICasm module will therefore attempt to RMFaster the BASIC module
which will require a small amount of memory in the RMA, in addition to that
required by the ExtBASICasm module itself. Attempting to run it while BASIC
is active and in ROM will not work - try "*RMFaster BASIC" at the BASIC
prompt and you'll see why.

  Ver    OS    Supported?
  1.05    3.1x    Yes
  1.06    3.5x    Yes
  1.14    3.6x    Yes
  1.16    3.7x    Yes
  1.17    3.8x    Yes
  1.18    4.00    No (1)
  1.19    4.01    No (1)
  1.20    4.0x    Yes

(1) Support for these versions will not be added since v1.20 is available for
    softload on RISC OS 4.


Enabling ExtBASICasm
————————————————————

Unlike some earlier versions, this version is initialised into a dormant
state whenever you start up the BASIC interpreter, eg. by double-clicking on
a BASIC program or by typing BASIC at the * prompt.

You can enable or disable the extensions by using the assembler pseudo-op
    EXT n
where n is 0 to disable and 1 to enable. (Other values are currently mapped
to 1; do not rely on this.)

Setting any of the extension OPT bits *NO* *LONGER* enables ExtBASICasm.

Certain extensions remain enabled at all times: specifically, ALIGN always
zero-fills, and the ".foo = bar" bug remains fixed. I don't think that
this'll inconvenience anybody :-)


ExtBASICasm uses the BASIC data word TIMEOF, which is documented as "unused"
for all versions of BASIC V which it recognises, for its 'enabled' flag. It
also uses the byte at &86E4 for its extended options byte.


The instructions added by the module are as follows:


Extensions
——————————

    Optional parts are enclosed in []

OPT    [<value>]
    Bit 4:    ASSERT control (1 = enabled on 'second pass')
    Bit 5:  APCS register names (1 = enabled)
    Bit 6:    UMUL/UMULL control (0 = short forms, 1 = long forms)
    Bit 7:    AOF control. If set, then the AOF extension is enabled.

    When ExtBASICasm is disabled, these bits take on their standard
    meanings: bit 4 allows use of, amongst others, the FP instructions.

    If value is omitted, the previous setting is used.

EXT    <flag>
    <flag>,<value>
    <flag>,
    Initialises or disables the extensions according to flag. The second
    form allows you to simultaneously set the OPT value; the third (a
    side effect of the use of the OPT code) causes the previous OPT value
    to be used.

ALIGN
    Zero-initialises the memory if required.

ALIGN    <const>[,<const2>]
    Aligns to a multiple of const bytes plus an optional offset. const
    must be a power of 2 between 1 and 65536; const2 must be between 0
    and const-1 (default is 0). Also zero-initialises the memory.
    P% becomes (P%+const-1 AND const-1)+const2; O% is also updated if
    necessary. Examples:
        ALIGN 4
        ALIGN 32
        ALIGN 16,8

MUL{cond}{S}    Rd,Rm,#<const>
    variable length; Rd=Rm if <2 ADD/RSB
    May cause 'duplicate register' if Rd=Rm and const is not simple - ie.
    not 0, (2^x)-1, 2^x, (2^x)+(2^y)

MLA{cond}{S}    Rd,Rm,#<const>,Ra
    variable length; Rd=Rm if <2 ADD/RSB
    Rd=Ra causes 'duplicate register' error if const is not simple, as
    for MUL; Rd=Rm=Ra is special in that MLA Rd,Rd,#c,Rd = MUL Rd,Rd,#c+1
    If Rd=Ra and const=0, no code is generated (none necessary).

DIV        Rq,Rr,Rn,Rd,Rt [SGN Rs]
    Integer division by register
    Rq = quotient        Rn = numerator        Rt = temporary store
    Rr = remainder        Rd = denominator    Rs = sign store
    If Rs omitted then division is unsigned.
    Rr may be same register as Rn *or* Rn may be same as Rs.
    All other registers must be different.
    Rt and Rs (if specified) are corrupted.

DIV        Rq,Rr,Rn,#d[,[Rt]] [SGN Rs]
    Integer division by constant
    Registers as above
    If Rs omitted then division is unsigned.
    If Rt omitted and is required for this division then error given.
    All registers must be different.
    If specified, Rt and Rs are corrupted.
    (Uses generator to build code - fast but may be long)
    Notes:    Uses Fourier method. For unsigned values, this is fixed to
        handle unsigned top-bit-set properly, *except* for div by 3
        which works for values up to &C0000000. Ideas and code
        gratefully received...

    *** Note no conditional for DIV

SQR        Rt,Rr,Rx,Ry
SQR        Rt,Rr,{Rx,Ry}
    Square root
    Input    Rr = value, Rx & Ry = work registers
    Output    Rt = square root, Rr = remainder, Rx = 0, Ry corrupt
        In effect, Rt = INT SQR Rr and Rr' = Rr-Rt*Rt
    If {} are used, then Rx and Ry are preserved via STMFD R13!,{Rx,Ry}
    and restored via LDMFD R13!,{Rx,Ry}.

    *** Note no conditional for SQR

ADR{cond}L    Rd,<const>
    Fixed length (two words)

ADR{cond}X    Rd,<const>
    Fixed length (three words)

ADR{cond}W    Rd,<const>
    Addressing relative to R12, one to three words
    <const> MUST be defined before it is used
    Adds/subtracts const to/from R12, storing result in Rd
    Up to you to ensure that R12 correctly set up...

LDR, STR
    xxx{cond}{B}W    Rd,<offset>
      Load/store word/byte at [R12,#<offset>]

    LDR{cond}{B}L    Rd,<address>
    LDR{cond}{B}L    Rd,[Rm,#<offset>]{!}
    LDR{cond}{B}WL    Rd,<offset>
    STR equivalents
      Addressing range is ±1MB; some offsets outside this range are also
      valid. Lengths are (in words):
        LDR           2  ADD/SUB Rd,Rm,#a:LDR Rd,[Rd,#b]
        LDR ...]!  2  ADD/SUB Rm,Rm,#a:LDR Rd,[Rm,#b]!
        STR           3  ADD/SUB Rm,Rm,#a:STR Rd,[Rm,#b]:SUB/ADD Rm,Rm,#a
        STR ...]!  2  ADD/SUB Rm,Rm,#a:STR Rd,[Rm,#b]!

    LDR{cond}{B}L    Rd,{Rn},<address>
    LDR{cond}{B}L    Rd,{Rn},[Rm,#<offset>]
    LDR{cond}{B}WL    Rd,{Rn},<offset>
    STR equivalents
      [{Rn} is NOT optional]
      Equivalent to the LDR/STRs above, except that Rn (rather than Rd)
      is used to hold the address; always two words long. For example,
      ADRL R0,wibble:LDR R1,[R0] may be replaced with LDRL R1,{R0},wibble
      - one word shorter.
      Rd=Rn is not allowed.
      Assembles to ADD/SUB Rn,Rm,#a : LDR/STR Rd,[Rn,#b]!

    LDR{cond}{B}L    Rd,[Rm],#<offset>
    STR equivalent
      Addressing range is ±1MB; some offsets outside this range are also
      valid. Two words long.
      Assembles to LDR/STR Rd,[Rm],#b:ADD/SUB Rm,Rm,#a

    NOTE: You should try to avoid using *sequences* of LDRLs or STRLs -
    there is usually a more efficient way.

    LDRxxH, LDRxxSH, LDRxxSB and STRxxH
        The W forms are supported.
        Long LDR{H|SH|SB} not yet implemented.

SWAP{cond}[S]    Rd,Rn
    Swaps Rd and Rn without using temporary store.
    Uses EOR method, is therefore three words long.
    If S is specified, then the flags are set according to Rn.

VDU{cond}{X}    <const>
    = SWI "OS_WriteI"+<const>
    With X present, XOS_WriteI is used instead.

NOP{cond}
    = MOV{cond} R0,R0
    (BASIC supports only an unconditional NOP.)

BRK{cond} [#<const>]
    Undefined instruction. If <const> is specified, then R14 is set to
    this value before the undefined instruction trap is taken.

EQUx, DCx, =
    xxx <value>[,<value>]^
    Extended form of EQUD, EQUW, DCB, etc.
    Instead of, eg. DCD 0 : DCD 12 : DCD branch
    you can now use DCD 0, 12, branch

Negative constants
    Allowed in the following instructions:
        ADD, SUB    ADC, SBC    ADF, SUF
        AND, BIC    MOV, MVN    MVF, MNF
        CMP, CMN    CMF, CNF    CMFE, CNFE
    If the constant is invalid for one of these, it is negated or
    inverted, as appropriate, and the instruction changed to the other of
    the pair (eg. ADC becomes SBC). If the constant is still invalid, the
    "bad immediate constant" error is generated as normal.


ARMv3 (ARM6) and later
——————————————————————

MSR{cond}    <psr>_<f>,Rm
    (Standard extension.)
    <f> may, in addition to the standard combinations of 'c' 'x' 's' 'f',
    be one of:
        ctl    control bits only
        flg    flag bits only
        all    both
    Any combination of 'cf' is equivalent to 'all'; you may also, in the
    standard form, use '_' between letters.


ARMv4 (ARM8, StrongARM) and later
—————————————————————————————————

UMUL, SMUL, UMLA, SMLA:
    xxx{cond}{S}    Rl,Rh,Rm,Rn

    The 'official' forms UMULL, SMULL, UMLAL, SMLAL are used *instead of*
    the 'short' forms if extended OPT bit 6 is set.
    Unfortunately it's not possible to allow both forms at once: how
    would you interpret "UMULLS" - UMUL condition LS or UMULL with S bit?


Floating-point instructions
———————————————————————————

Floating point coprocessor data transfer

LDF, STF:
    xxx{cond}precW    Fd,<offset>

LFM{cond}{stack}    Fd,m,[Rn]{!}
SFM{cond}{stack}    Fd,m,[Rn]{!}
LFS{cond}{stack}    Rn{!},<fp register list>
LFS{cond}{stack}    Rn{!},<fp register list>

    LFM, SFM, LFS and SFS use extended precision. The <fp register list>
    is much as for LDM and STM, with restrictions: you must specify a
    register or a sequence of registers, and the list must be compatible
    with LFM and SFM - eg.
    LFSFD R13!,{F3}        LFMFD F3,1,[R13]!    LFM F3,1,[R13],#12
    SFSFD R13!,{F5-F0}    SFMFD F5,4,[R13]!    SFM F5,4,[R13,#-36]!
    LFSDB R13,{F1,F0}    LFMDB F0,2,[R13]    LFM F0,2,[R13,#-24]
    - for each row, all the instructions have the same effect.
    Available stack types are DB, IA, EA, FD.
    Note that example 2 wraps around - F5, F6, F7, F0 _in that order_.


Assembler directives
————————————————————

* Conditional - will STOP if expression is FALSE:

ASSERT    <expression>

    Bit 4 of the extended OPT value controls ASSERT. When it and bit 1
    are zero, ASSERTs are ignored.

* Constants

=    <const|string>
    The bug causing an error when used in the form
        .label = "something"
    has been fixed.

EQUF, DCF
    xxx    <const>

    Synonyms for EQUFD.

EQUP, DCP, P
    xxx    <string>,<const>
    xxx    <const>,<string>
    Fixed-length string allocation. If the string is too short, then the
    remaining space is padded with nulls; if it is too long, it is
    truncated to the specified length.

EQUPW, DCPW, PW
    xxx    <pad_byte>,<string>,<const>
    xxx    <pad_byte>,<const>,<string>
    Like EQUP, except that you specify the padding byte.

EQUZ, DCZ, Z
    xxx    <string>
    EQUS with automatic zero termination

EQUZA, DCZA, ZA
    xxx    <string>
    Equivalent to EQUZ followed by ALIGN

    Note: *ALL* the EQU... directives (and their equivalents) may have
    their arguments repeated as described in the Extensions section.

FILL, %
    xxx{B|W|D}    <const>{,{<value>}}
    Allocates <const> bytes of memory, initialised to <value> (or 0).
    B, W and D represent data lengths as for EQU; if omitted, then byte
    length is assumed. If the comma is present but no fill value, this is
    equivalent to adding the constant to P% (and O% if appropriate).

FILE    <filename>
    Loads the specified file, allocating just enough space for it.

^    <offset>
    Initialises the workspace address pointer to the given value.
    This is used and updated by #.
    Typical use:
        ^ 0
        ...
        # flags, 4
        ...
        LDRW    R0,flags

#    <variable>, <length>
    Sets the variable to the current value of the workspace address
    pointer, which is then incremented by <length>.
    This does not alter P% or O%.
    (Note: the variable is assigned before the length is evaluated.)

COND    <cond>
    Sets the condition code for use with = (when used as a condition
    code). It may be supplied as a condition code literal, a number (0 to
    15), or a string containing a condition code literal. For example,
    all of the following are equivalent:
        COND    7        ; number
        COND    VC        ; condition code literal
        COND    vc        ; condition code literal
        COND    "Vc"        ; string containing cond. code lit.
    Example code:
        COND    LT        ; select LT condition code
        MOV=    R0,#2        ; MOVLT  R0,#2
        MOV=S    R1,R2        ; MOVLTS R1,R2


AOF generation
——————————————

AREA    "name", "attributes"

    An area is a named block of code or data that can be manipulated by a
    linker, such as drlink, to form the final program image. Typically, a
    program will be divided into two main areas; one for code and the
    other for data. For example, Acorn's C compiler uses areas named
    "C$$code" and "C$$data".

    Each area has a set of attributes which provide extra information to
    the linker. The attributes recognised by ExtBASICasm are listed below
    (the case is ignored):

    32BIT        The code complies to the 32-bit variant of the APCS.
    CODE        Contains machine code instructions.
    DATA        Contains data, not instructions.
    EXTFP        The extended floating-point instruction set is used (LFM
            and SFM instead of multiple LDFEs and STFEs).
    NOCHECK        The code complies with a variant of the APCS without
            software stack-limit checking.
    PIC        Position Independent Code; will execute where loaded
            without modification.
    READONLY    This area will not be written to.
    REENTRANT   The code complies with the reentrant APCS standard.

    Each attribute should be separated from the next by a comma (spaces
    are optional and ignored when processing the list). It is important
    to make sure that all areas of the same name also have the same
    attributes, otherwise the linking process will fail.

    Example area definitions:

    AREA    "C$$code", "CODE, READONLY"
    AREA    "MyProgram$Data", "DATA"


IMPORT    "symbol" ["attributes"]
IMPORT    "symbol", "alternative name" ["attributes"]

    References between areas are handled via "symbols". Each area exports
    a list of symbols that are available for external use, eg "strlen",
    "printf", and "fread" from the C library stubs. An area can then
    import the symbol and use it as if it were defined locally, leaving
    the linker to resolve the references later.

    IMPORT makes an external symbol available for use within the current
    area. It creates a variable of the same name, ending with an @, which
    can be used with STR, LDR, EQUD, EQUW and B instructions. An
    alternative name can be supplied, making it possible to import
    symbols containing characters that are illegal in a BASIC variable
    name, such as $. Example uses:

    IMPORT  "strcpy"        ; import strcpy as strcpy@
    IMPORT  "an$example", "example" ; import an$example as example@

    BL    strcpy@            ; call the strcpy routine
    LDR    R0, example@        ; load a word from an$example

    Note: The '@' variables should always be treated as read-only.

    The optional attributes are as follows:

    FPREG        This is only meaningful if the imported symbol is a
            function entry point and indicates that floating point
            arguments will be passed in floating point registers.
    INSENSITIVE The linker will ignore the case when trying to resolve
            the reference.
    NOCASE        A synonym for INSENSITIVE.
    WEAK        It is acceptable for the reference to remain unsatisfied.

    Example:

    IMPORT  "xyz", "abc" ["nocase"] ; import xyz as abc@, ignoring case


EXPORT    "symbol" ["attributes"]
EXPORT    "symbol", "alternative name" ["attributes"]

    EXPORT makes an address within the current area available for outside
    use. The "symbol" is the BASIC variable containing the address and,
    if the alternative name is missing, the name under which it is
    exported. If the variable is an integer you would probably want to
    supply an alternative name to remove the % at the end. Example uses:

    EXPORT  "compare%", "uint_cmp"  ; export compare% as uint_cmp
    EXPORT  "value", "an$integer"    ; export value as an$integer
    EXPORT  "value"            ; also export value as value

    .compare%
    CMP    R0, R1
    MOVLO    R0, #-1
    MOVEQ    R0, #0
    MOVHI    R0, #1
    MOVS    PC, R14
    .value
    DCD    &12345678

    The optional attributes are:

    DATA    This is only meaningful if the symbol occurs in a code area,
        and indicates that the symbol defines data rather than code.
    FPREG    This is only meaningful if the symbol defines a function
        entry point and indicates that floating point arguments
        should be passed in floating point registers.
    LEAF    The symbol defines a function call that makes no calls to any
        any other functions.
    STRONG    This symbol should be used in preference to any other
        non-strong symbol when resolving references in other files.
        Any references to the symbol within the same file as the
        strong definition resolve to the non-strong definition. This
        allows a kind of link-time indirection.

    Example:

    EXPORT  "leaf_fn" ["leaf"]    ; export leaf_fn as a leaf symbol


INCLUDE    "filename"

    INCLUDE will load the specified BASIC program as a library (first
    pass only) and call the definition of FNinclude in that program, if
    one exists. The function call ignores definitions of FNinclude
    elsewhere, such as other INCLUDE'd files.


END
SAVE    "filename"

    It is necessary to explicitly mark the end of an AOF file, to allow
    the extra data to be inserted in the correct place, and to provide a
    means of determining how many passes have been carried out. Either
    END or SAVE "filename" can be used for this purpose; the latter will
    automatically save the AOF output on the second pass. For example:

    SAVE    "o.program"


LDR    register, =expression
LDF*    register, =expression
LTORG

    Literals are used to load immediate values that cannot be handled by
    the MOV/MVN and MVF/MNF instructions. The expression is evaluated and
    stored in the nearest following literal pool, and the instruction is
    assembled to load from the value from this address. If the expression
    contains an imported symbol, the necessary relocation directive will
    be transparently added.

    A literal pool is automatically added at the end of each area, but
    extra literal pools can be created using the LTORG directive. This is
    particularly useful when using floating point literals as the LDF
    instruction only has a range of +-1020.

    Examples:

    LDR    R0, =&4b534154        ; R0 = &4b534154 ("TASK")
    LDFS    F1, =3.1415926536    ; F1 = (float) 3.1415926536
    LDR    R7, =external@ + 4    ; R7 = address of external + 4


HEAD    "function name"

    This adds a function name header, used by backtraces and
    disassemblers to name functions. For example:

    EXPORT  "compare"
    HEAD    "compare"
    .compare
    ; ...
    MOVS    PC, R14


ENTRY

    The ENTRY directive is used to tell the linker where program
    execution should begin.


ORG    address

    ORG sets a base address for the current area. It should be used
    carefully as it may cause problems for the linker, and only really
    makes sense if the code needs to be mapped to a fixed hardware
    location.


ROUT
ROUT    "routine name"

    ROUT marks the beginning of a new local label block. Local labels can
    be defined multiple times in a single source file and are
    particularly useful in macros. For example:

    DEF FNexample
    [    OPT    pass%
        ROUT
        TST    R0, #1
        BNE    @10
        ; ...
        B    @20
    .10
        ; ...
    .20
    ]
    = 0

    A local label always starts with a number and can optionally be
    followed by the routine name, as supplied to the preceding ROUT. It
    is an error to supply differing names; local labels outside the
    current label block are hidden. References to a local label begin
    with an @.


Notes
—————

* Registers are specified in the following form:

    ARM registers:            R0..R15
        using APCS-R names:    A1..A4 V1..V6 SL FP IP SP LR PC
    Floating-point registers:    F0..F7
    General co-processor registers:    C0..C15

  To help to cope with any potential name clashes, the floating point and
  APCS-R register names (except for PC) must be terminated with some
  character not valid in a variable name in order to be recognised; they are
  otherwise treated as part of a variable name.

* Coprocessor numbers (CP#) may be specified using either of the following
  forms:

    P0..P15
    CP0..CP15

* Wherever a register or coprocessor number is specified, an expression may
  be substituted in the usual manner allowed by BASIC V. This module employs
  the routines used within BASIC to evaluate all expressions (eg. register
  numbers, offsets and labels) and hence its interpretation of expressions is
  guaranteed to be the same as BASIC.


Credits
———————

  Adrian Lees (last known, AFAIK, at A.M.Lees-CSEE93@@cs.bham.ac.uk):
  - for the original ExtBas and the EQU comma extension, and for the use of
    some of his code

  Michael Rozdoba ([e-mail]; remember TechForum?):
  - for including the "General recursive method for Rb := Ra * C, C a
    constant" from Appendix C of the manual for Acorn's desktop assembler,
    and the late Acorn Computing (Sept 1994) for printing it;
  - for the division code generator (Archimedes World, May 1995), which was
    included, slightly trimmed, and debugged to handle top-bit-set unsigned
    numbers properly... I hope!

  Dominic Symes of !Zap fame ([e-mail]):
  - for pointing out that ANDEQ R0,R0,R0 could usefully be replaced by DCD 0

  Martin Willers ([e-mail]):
  - for bug hunting :-)

  Reuben Thomas ([e-mail]):
  - for pointing out it might be useful to disable the APCS register names,
    suggesting B/W/D suffix for FILL (and %) and -ve immediate constants, and
    bug encountering

  Mohsen Alshayef ([e-mail]):
  - for some useful long MUL, STRH and [CS]PSR info

  Michael Kircher ([e-mail]):
  - for the integer square root code, and a few bug reports