Prev: 2 Distribution

3 Model Organization

The fig-FORTH model deviates a bit from the usual loading method of Forth. Existing systems load about 2k bytes in object form and then self-compile the resident system (6 to 8 k bytes). This technique allows customization within the high level portion, but is impractical for new implementors.

Our model has 4 to 5 k bytes written as assembler listings. The remainder may be compiled typing in the Forth high-level source, by more assembly source, or by disc compilation. This method enhances transportability, although the larger portion in assembly code entails more effort. About 8k bytes of memory is used plus 2 to 8k for workspace.

3.1 MODEL OVERVIEW

The model consists of 7 distinct areas. They occur sequentially from low memory to high.

Boot-up parameters Machine code definitions High level utility definitions Installation dependent code High level definitions System tools (optional) RAM memory workspace

3.2 MODEL DETAILS

Boot-up Parameters

This area consists of 34 bytes containing a jump to the cold start, jump to the warm re-start and initial values for user variables and registers. These values are altered as you make permanent extensions to your installation.

Machine Code Definitions

This area consists of about 600 to 800 bytes of machine executable code in the form of Forth word definitions. Its purpose is to convert your computer into a standard Forth stack computer. Above this code, the balance of Forth contains a pseudo-code compiled of "execution-addresses" which are sequences of the machine address of the "code-fields" of other Forth definitions. All execution ultimately refers to the machine code definitions.

High-level Utility Definitions

These are colon-definitions, user variables, constants, and variables that allow you to control the "Forth stack computer". They comprise the bulk of the system, enabling you to execute and compile from the terminal. If disc storage (or a RAM simulation of disc) is available,you may also execute and compile from this facility. Changes in the high-level area are infrequent. They may be made thru the assembler source listings.

Installation Dependent Code

This area is the only portion that need change between different installations of the same computer cpu. There are four code fragments:

(KEY)

Push the next ascii value (7 bits) from the terminal keystroke to the
computation stack and execute NEXT. High 9 bits are zero. Do not echo
this character, especially a control character.

(EMIT)

Pop the computation stack (16 bit value). Display the low 7 bits on the
terminal device, then execute NEXT. Control characters have their
natural functions.

(?TERMINAL)

For terminals with a break key, wait till released and push to the
computation stack 0001 if it was found depressed; otherwise 0000.
Execute NEXT. If no break key is available, sense any key depression as
a break (sense but don't wait for a key). If both the above are
unavailable, simply push 0000 and execute NEXT.

(CR)

Execute a terminal carriage return and line feed. Execute NEXT.

When each of these words is executed, the interpreter vectors from the definition header to these code sequences. On specific implementations it may be necessary to preserve certain registers and observe operating system protocols. Understand the implementors methods in the listing before proceeding!

R/W

This colon-definition is the standard linkage to your disc. It requests the read or write of a disc sector. It usually requires supporting code definitions. It may consist of self-contained code or call ROM monitor code. When R/W is assembled, its code field address is inserted once in BLOCK and once in BUFFER.

An alternate version of R/W is included that simulates disc storage in RAM. If you have over 16 k bytes this is practical for startup and limited operation with cassette.

High-level Definitions

The next section contains about 30 definitions involving user interaction: compiling aids, finding, forgetting, listing, and number formating. These definitions are placed above the installation dependent code to facilitate modification. That is, once your full system is up, you may FORGET part of the high-level and re-compile altered definitions from disc.

System Tools

A text editor and machine code assembler are normally resident. We are including a sample editor and hope to provide Forth assemblers. The editor is compiled from the terminal the first time, and then used to place the editor and assembler source code on disc.

It is essential that you regard the assembly listing as just a way to get Forth installed on your system. Additions and changes must be planned and tested at the usual Forth high level and then the assembly routines updated. Forth work planned and executed only at an assembly level tends to be non-portable, and confusing.

RAM Workspace

For a single user system, at least 2k bytes must be available above the compiled system (the dictionary). A 16k byte total system is most typical.

The RAM workspace contains the computation and return stacks, user area, terminal input buffer, disc buffer and compilation space for the dictionary.

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