Streams and Channels

This program is a diagnostic utility that displays the contents of the system variable STRMS (streams table) at addresses 23568–23605, showing each two-byte word as a 16-bit little-endian value alongside its channel label (−3 through 15). It then reads the CHANS system variable (from addresses 23631–23632) to locate and display the 21-byte channel definition table in RAM, printing each entry’s output routine address, input routine address, and device specifier character. The program uses OVER 1 printing and PLOT/DRAW to create an overlay grid illustrating the table layout. A reference page lists the standard device specifier codes (75=K keyboard, 83=S screen, 82=R RAM write, 80=P printer), and the utility includes options to copy output to the printer by poking channel 0’s device byte with 80 (printer) or restoring it with 83 (screen). The program explicitly notes it runs on both the standard TS2068 ROM and the TS2068 with a Spectrum ROM, with different CHANS table locations between the two configurations.

---

Program Analysis

Program Structure

The program is organized into five logical sections separated by keypress pauses that optionally trigger a printer copy:

1. Lines 110–315: Loop over the STRMS system variable area (23568–23605), printing each 16-bit word and its channel label (−3 through 15).
2. Lines 315–361: Pause/copy prompt, then display a device specifier reference page (K, S, R, P codes).
3. Lines 401–521: Read CHANS pointer, walk the 21-byte channel table, display output/input routine addresses and device bytes, then draw a graphic overlay with OVER 1 and PLOT/DRAW.
4. Lines 530–562: Compatibility note and printer-poke instructions, final menu offering Save, Copy, Run, or Stop.
5. Lines 9997–9998: STOP and SAVE stub.

STRMS Table Display (Lines 110–310)

The FOR loop steps by 2 from 23568 to 23605, covering 19 two-byte STRMS entries (channels −3 through 15). Each 16-bit pointer is read with the standard little-endian idiom:

PEEK N + PEEK (N+1) * 256

Rather than using a computed channel label, each address is matched with a dedicated IF at lines 120–300 — one line per channel. This is verbose but ensures exact label strings including the aligned spacing for negative channel numbers and double-digit channel numbers.

CHANS Table Display (Lines 401–510)

The CHANS system variable pointer is read at line 401:

LET C = PEEK 23631 + PEEK 23632 * 256

Lines 410–450 then manually walk four complete 5-byte channel entries plus the terminating byte. Each entry consists of:

• 2 bytes — output routine address (printed as a 16-bit word)
• 2 bytes — input routine address (printed as a 16-bit word)
• 1 byte — device specifier character (printed as both numeric and CHR$)

The variable C is advanced manually between entries with LET C = C + 3 (stepping past the two bytes already consumed by the inner FOR loop plus the device byte). This pointer arithmetic is done entirely in BASIC without any machine code assistance.

OVER 1 Overlay Technique (Lines 460–510)

After printing the CHANS table data, lines 460–510 use OVER 1 to XOR-draw column separator lines and header labels over the already-printed content. The sequence is:

• Line 460: OVER 1 enabled
• Line 470–500: Column headers (“OUTPUT ROUTINE ADR”, “INPUT ROUTINE ADDR”, “DEVICE SPEC”) printed four times at successive rows using PRINT AT and TAB
• Line 511: A box drawn with PLOT/DRAW at pixel coordinates to bracket the table area
• Line 510: OVER 0 to restore normal mode

This produces a visual grid aligned with the printed data without erasing it — a technique that depends on the precise character positions used in the earlier PRINT statements.

Pause/Copy/Print Idiom (Lines 315, 360, 520, 560)

Each section ends with a consistent pattern printed to stream 1 (the lower information line):

PRINT #1;AT 0,0;"ANY KEY TO CONTINUE, Z TO COPY": PAUSE 0: IF INKEY$="Z" THEN COPY : CLS

The PAUSE 0 followed by INKEY$ is the standard efficient keypress-wait idiom. The COPY command sends the current screen to the printer. Note that after COPY the program falls through to the next line (CLS) rather than branching away, so the screen is cleared either way.

Printer Redirect Information (Lines 530–540)

The program documents a useful technique: poking the device specifier byte of channel 0 with 80 (ASCII “P”) redirects all PRINT output to the printer. The correct address differs between ROM configurations:

Configuration	POKE Address	Value for Printer	Value to Restore
TS2068 native ROM	26697	80	83
TS2068 with Spectrum ROM	23743	80	83

This works because the CHANS table is located at different addresses under the two ROMs, but the device specifier byte within the screen channel entry is in both cases reachable by these absolute addresses.

Notable Anomalies and Observations

• Line 315 has a formatting issue: COPY : CLS is on the same line as the condition, meaning CLS executes unconditionally if “Z” is pressed (after COPY), but if any other key is pressed the line simply falls through to line 316 which is also CLS — so CLS always runs. This is intentional cleanup, not a bug.
• Line 450 has a stray single-quote inside a string literal: PRINT '"1 ";C;... — the ' before "1 " is a newline token outside the string, so this prints a blank line then the terminator byte entry. Correct behavior.
• The final menu at line 560 checks INKEY$ at lines 561–562 after the PAUSE 0 has already consumed one keypress at line 560. Unless the key is held down, lines 561–562 will typically read an empty string, making the “R” and “S” options unreliable. This is a minor design flaw.
• Line 562 does CLEAR : GO TO 9998 to reach the SAVE line, which erases all variables before saving — a common pattern to produce a clean tape file.