SEND VARS

SEND-VARS is a utility designed to work alongside the MTERM modem program, allowing machine code and BASIC variables to be transmitted over a modem link on the TS2068. The program uses direct memory POKEs to addresses in the 26700s range to invoke small machine code routines that manipulate the variable area, converting it to a modem-transmittable form and later restoring it. Line 2 accepts a code length and source address, computes high and low bytes via integer arithmetic, and stores them before calling USR 26735 to copy the target block into the string variable x$. Lines 3 and 4 use RANDOMIZE USR calls to hide and reveal variables respectively, a technique that temporarily makes the BASIC variable area invisible to the interpreter. Line 1 contains an embedded machine code payload stored within a REM statement, a common approach for self-contained TS2068 utilities that need resident routines without requiring a separate binary file.

Program Analysis

Program Structure

The program is a compact eight-line utility. Lines 1 and 7–8 are REM statements; line 1 holds the embedded machine code payload, while lines 7 and 8 provide user documentation. Lines 2–6 are the operational entry points, each intended to be invoked manually with a GO TO command rather than run sequentially.

Machine Code in the REM Statement

Line 1 is a REM statement whose body encodes a series of machine code routines. The BASIC interpreter ignores REM content at runtime, making it a safe container for arbitrary binary data. The routines are accessed via USR with hard-coded addresses in the 26715–26777 range, which point into the body of this REM line in memory. This is a well-established technique for embedding self-contained machine code in a BASIC program without a separate binary file.

The specific USR entry points used are:

• USR 26715 — hide variables (called from line 3)
• USR 26728 — reveal/restore variables (called from line 4)
• USR 26735 — copy source code block into x$ (called from line 2)
• USR 26754 — write x$ contents back to high memory (called from line 5)
• POKE 26777 / POKE 26776 — pass parameters (high byte and low byte of source address) to the machine code

Address Calculation Technique

Line 2 splits the 16-bit source address into its high and low bytes using a standard BASIC idiom: INT(sad/256) yields the high byte, and 256*(sad/256-INT(sad/256)) recovers the fractional part and scales it back to give the low byte. These are then POKEd into fixed locations (26777 and 26776 respectively) so the machine code routine can read them as a two-byte address parameter.

Variable Hiding Mechanism

The hide/reveal pair at lines 3 and 4 exploits the BASIC system variables that record the start or length of the variable area. By calling RANDOMIZE USR 26715, the machine code presumably moves or masks a system pointer so the interpreter no longer finds or displays the variable block — making the variables invisible to a casual LIST or inspection. RANDOMIZE is used rather than a plain USR expression to discard the return value cleanly. The subsequent STOP on each line prevents accidental fall-through.

The MERGE Workflow

The intended usage described in lines 7–8 is to MERGE this utility into an already-loaded BASIC program. Because MERGE adds lines without erasing existing ones, the target program’s lines and variables remain intact while the utility’s lines (1–8) are added. The user then executes specific GO TO targets (3, 4, or 2) by hand to trigger each phase of the encode/send/decode cycle.

Notable Techniques and Idioms

• Machine code stored in a REM statement body, accessed by absolute USR address — requires the program to always load at the same memory location.
• RANDOMIZE USR n used to call machine code and suppress the return value without a LET assignment.
• High/low byte decomposition of a 16-bit address performed entirely in BASIC floating-point arithmetic.
• DIM x$(col) allocates a string of exactly the required length to hold the binary code payload.
• LET x$(1)=CHR$ USR 26735 — the assignment to x$(1) is a side-effect trigger; the machine code routine uses the pre-POKEd parameters and fills x$ in bulk, returning an irrelevant character to satisfy the BASIC expression.
• Line 5 uses IF sad AND col as a guard to ensure both parameters were set before attempting to replace code, providing minimal input validation.

Potential Issues

The machine code entry points are hard-coded absolute addresses. If the program is loaded at a different memory location (e.g., due to a different BASIC program preceding it in memory), all USR calls will point to wrong or garbage memory, causing a crash. The utility is therefore fragile and must be loaded fresh or MERGEd in a controlled way that guarantees its position in RAM. No bounds checking is performed on the user-supplied col or sad values before the POKEs and USR calls are made.

    1 REM *K\"\gh*Y\"K\<>*\gh"K\<>*M\T]+F+N*\ih GO SUB VAL *M\N<>*M\ RESTORE +F+N LLIST GO SUB [\ih GO SUB VAL *M\N<>\d| OPEN # 
    2 INPUT "Code length? ";col: INPUT "Source address? ";sad: DIM x$(col): POKE 26777,INT (sad/256): POKE 26776,256*(sad/256-INT (sad/256)): LET x$(1)=CHR$ USR 26735: REM  line puts code in x$
    3 RANDOMIZE USR 26715: STOP : REM  line hides variables      
    4 RANDOMIZE USR 26728: STOP : REM  line reveals variables    
    5 IF sad AND col THEN LET x$(1)=CHR$ USR 26754: STOP :             REM  line replaces code   
    6 STOP 
    7 REM    SEND-VARS            An MTERM MODEM program utility        \* 1984 David Hoshor      TO USE SEND-VARS WITH A BASIC   PROGRAM, LOAD BASIC PROGRAM AND/OR VARIABLES THAT YOU WANT TO   SEND BY MTERM MODEM PROGRAM.    MERGE SEND-VARS. ENTER: GO TO 3.THE VARIABLES WILL BE CONVERTED TO A MODEM SENDABLE FORM. WHEN  THE RECIPIENT GETS THE PROGRAM, HE SHOULD ENTER: GO TO 4. THE   VARIABLES WILL NOW BE RESTORED. 
    8 REM TO SEND CODE FROM HIGH MEMORY ENTER: GO TO 2. YOU WILL BE PROMPTED FOR LENGTH AND      ADDRESS OF CODE. THE CODE WILL  BE MOVED INTO X$ AND VARIABLES  CONVERTED TO SENDABLE FORM.     WHEN RECEIVED THE RECIPIENT MUSTENTER: GO TO 4 THE CODE IN X$   WILL BE REPLACED IN HIGH MEMORY.