245 lines
10 KiB
Plaintext
245 lines
10 KiB
Plaintext
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pfth Version 1.02
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August 19, 2013
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Dave Hein
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(with additions from G. Herzog)
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INTRODUCTION
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------------
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pfth is an ANS Forth interpreter that runs on the Propeller. It is written in
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PASM and Forth, and it can be built using the Prop Tool or BST. pfth
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implements all 133 of the ANS Forth core words, and 38 of the 45 core ext
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words. pfth will run on any Propeller board that supports the standard serial
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interface. The default settings of the serial port are 115200 baud, 8 bits, no
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parity and 1 stop bit.
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After loading and communications is established, use 'words' to verify that the
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display is correct. The CR word is defined to emit both a carriage return and
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a line feed. If your terminal displays an extra line you can either disable
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the linefeed character on your terminal, or re-define CR to only emit a
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carriage return.
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VERSIONS OF SPIN FILES
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----------------------
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There are four versions of pfth.
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The first version can be used to build stand-alone Forth applications. It is
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in pfth.spin. In this version, Forth programs are included using the Spin FILE
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directive.
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A second version interfaces to an SD card and can execute Forth programs on
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the SD card. This version is in sdpfth.spin. The program is set up for the C3
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card, but it can be modified to support other cards.
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The third version is named ospfth.spin, and runs under the Spinix operating
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system. When Spinix starts up pfth it provides information about the SD pins,
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the current working directory and a parameter list. The OS version of pfth
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uses this information to initialize the SD card driver and change to the
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working directory. It includes the file given by the parameter list.
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The final version is called chess.spin, and implements a chess program. After
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the program is loaded type "chess" to start playing. A move is entered by
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typing the source and destination postions separated by a "-". As an example,
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the move "D2-D4" will move the white queen's pawn two spaces ahead.
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LEXICON
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-------
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pfth has over 50 kernel words, with most of them from the ANS core word set.
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The source code for pfth is contained in pfth.spin, init.fth and other Forth
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programs included at the end of pfth.spin.
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Two of the non-standard words implemented by pfth are cog@ and cog!. These are
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are used to read and write cog memory locations. Their main purpose is to
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access the Prop registers, such as CNT, INA and OUTA.
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Another non-standard word is cogx1, which is used to execute native Prop
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instructions. The TOS contains the Prop instruction, and the next value on the
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stack is used for the destination register value. The result of the execution
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of the instruction is returned on the stack.
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Some other non-standard words are _lit, _gethex, _jz and .x.
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_lit is used to encode literal values.
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_gethex is used during the boot phase to convert numeric strings to hex values.
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_jz implements a jump-on-zero primitive.
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.x is used for debug purposes, and it prints values as 8-digit hex numbers.
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Some of the kernel words are shown below.
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Interpreter Words Math and Logical Words Memory Access
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----------------- ---------------------- -------------
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evaluate + !
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execute - @
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find * c!
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word / c@
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refill mod
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create and Console I/O
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: or -----------
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; xor emit
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< key
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= accept
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Program Termination >
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------------------- lshift Primitive Words
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abort rshift ---------------
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exit _lit
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quit Variables _gethex
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--------- _jz
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Stack Operations #tib .x
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---------------- tib
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drop >in Propeller Words
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pick base ---------------
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roll dp cog!
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>r last cog@
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r> state cogx1
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depth
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swap
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dup
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pfth also contains a small number of pre-compiled Forth words written
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in Forth. These words are here, allot, ",", _jmp and count. The definition
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of these words is as follows.
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: here dp @ ;
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: allot dp @ + dp ! ;
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: , here ! 4 allot ;
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: _jmp r> @ >r ;
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: count 0 pick 1 + 1 roll c@ ;
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AT START UP
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-----------
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When pfth starts up, it runs a small boot interpreter that compiles
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the ANS dictionary contained in init.fth. This file is included in the
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PASM binary image using the PASM FILE directive. Other Forth source files
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may be included after init.fth to add additional words and to implement a
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specific application.
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The boot interpreter can only handle 16-bit hex numbers, so pfth is in the hex
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mode when first starting up. The boot interpreter has no error handling
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capability, and is only used to build the initial dictionary. The boot
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interpreter uses a limited vocabulary consisting of the following 20
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kernel words.
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dp state _lit _gethex : ; c@ @ ! + = pick roll drop
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r> >r word find execute refill
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The boot interpreter is shown below in a pseudo-Forth language. The
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labels (label1), (label2), etc. are actually encoded as PASM labels, but
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they are shown symbolically in the code below.
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: xboot
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(label1)
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20 word 0 pick c@ _jz (label2) ( Get word, refill if empty )
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find 0 pick _jz (label3) ( Find word, get number if not found )
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state @ = _jz (label4) ( Go execute if not compile mode or immediate )
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, _jmp (label1) ( Otherwise, compile and loop again )
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(label4)
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execute _jmp (label1) ( Execute and loop again )
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(label3)
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drop count _gethex ( Get number )
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state @ _jz (label1) ( Loop again if not compile mode )
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_lit _lit , , _jmp (label1) ( Otherwise, compile number and loop again )
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(label2)
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drop refill _jmp (label1) ( Refill and loop again )
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;
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The boot interpreter compiles init.fth, which then runs the main interpreter.
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The main interpreter performs some error handling by checking for undefined
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words and stack underflows. It also handles negative numbers and numbers in
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any base.
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SOURCE PROGRAMS
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---------------
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There are a number of additional Forth source programs included in this
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distribution that can be run by pfth. Open and read these as text files to
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learn more details.
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comus.fth - provides a several useful non-standard words that are commonly
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used.
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starting.fth - contains samples of code from the "Starting Forth" tutorial.
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rc4time.fth - implements the RC4 code included in the Wikipedia Forth entry.
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It also displays the time required to run the RC4 code.
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i2c.fth - is a Forth implementation of Mike Green's basic_i2c_driver from
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the OBEX.
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fds.fth - implementes some of the functions from the FullDuplexSerial
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driver. It uses the binary PASM code from FullDuplexSerial.
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toggle.fth - will start up a cog running the Forth interpreter. It toggles
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P15, but it can easily be modified to toggle another pin.
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ted.fth - a simple line-oriented text editor based on the ED text editor.
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linux.fth - implements some basic linux commands, such as ls, cat, rm and cp.
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STARTING FORTH COGS
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-------------------
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Forth cogs are started with cognew or coginit by specifying the Forth cog image
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as the execution pointer and a 5-long structure as the data pointer. The
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5-long structure is defined as follow
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First long: Address of the body of a word that will be executed on startup
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Second long: Initial value of the stack pointer
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Third long: Address of the beginning of the stack
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Fourth long: Initial value of the return stack pointer
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Fifth long: Address of the beginning the the return stack
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CELL SIZE
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---------
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The cell size for pfth is 32 bits. The words !, @ and "," words access 32-bit
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values that are 32-bit aligned. Additional words are provide for smaller unit
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access, such as w! and w@ for word access, and c! and c@ for byte access.
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The compiled list cell size is 16 bits. The compile, word must be used when
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compiling execution tokens into a list rather than just using the "," word.
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DICTIONARY ENTRY FORMAT
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-----------------------
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The format for the pfth dictionary entry is shown below. The beginning of a
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word entry and its body are long-aligned. The link pointer contains the
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address of the previous word in the dictionary.
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The flags byte contains various flag bits that indicate if the word is an
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immediate word, a literal number or string, or a jump word. The name length
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byte and the name string specify the name of the word. It is followed by
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padding bytes that ensure long alignment.
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The code pointer contains the cog address of the PASM code that is to be
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executed. The execution token for a word points to this field. The does>
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pointer contains the hub address of a list of execution tokens that is to be
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called.
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The body is a variable length field that contains the contents of a variable or
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the list of execution tokens for a compiled word. The list for a compiled word
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consists of one execution token per word, and is terminated by a zero-valued
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word.
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Offset Content Size
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------ ------- ----
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0 Link Pointer word
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2 Flags byte
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3 Name Length byte
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4 Name String Len bytes
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4+Len Padding (-Len) & 3 bytes
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4+(Len+3)&(-4) Code Pointer word
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6+(Len+3)&(-4) DOES> Pointer word
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8+(Len+3)&(-4) Body Variable
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