DemoPeek

This program is a memory inspection utility that sequentially reads and displays the contents of any address in the ZX81/TS1000’s 64KB address space, showing both the decimal PEEK value and the corresponding CHR$ representation for each byte. It runs in SLOW mode and uses SCROLL to continuously advance through memory one byte at a time. Keyboard controls allow the user to input a new start address (A), return to BASIC (B), pause the display (M using PAUSE 4E4 for approximately 60 seconds), save memory (S), or jump to a main program menu (P). The SAVE routine at line 9992 defaults to the filename “PEEK” if no name is entered, and USR 8297 is called after saving to report remaining free memory bytes.

---

Program Analysis

Program Structure

The program occupies lines 9967–9999, clearly designed as a subroutine module at the top of the line-number space within a larger BASIC program. It begins with a STOP at line 9967 (a guard against falling into the help screen from above), then displays a help/instruction screen before entering the main memory-display loop. There are three functional sections:

1. Help screen and setup (9967–9978): Prints instructions and inputs the starting address into variable P.
2. Main display loop (9980–9988): Scrolls through memory one byte at a time, polling INKEY$ for control keys.
3. Save routine (9992–9999): Prompts for a filename, defaults to “PEEK”, performs SAVE, and reports free memory.

Main Display Loop

The core loop at lines 9980–9988 prints the current address value P, a tab, the result of PEEK P, and CHR$ PEEK P on one line, then calls SCROLL and increments P before re-entering the loop. This produces a continuously advancing listing of memory contents. Because there is no PAUSE or delay in the base loop, the display scrolls at full SLOW-mode speed.

Key BASIC Idioms and Techniques

• SLOW at line 9975 switches to SLOW mode to make the scrolling readable, as the ZX81 display would otherwise be suppressed during FAST-mode computation.
• PAUSE 4E4 at line 9985 uses scientific notation for the pause duration (40000 frames ≈ 10 minutes) as a “freeze” function.
• GOTO MENU at lines 9986 and 9999 targets a named (non-numeric) label — this is a GO TO targeting a variable whose value is a line number, a common technique for integrating sub-modules into a larger program without hardcoding jump targets.
• The INPUT P at line 9978 with a bounds check IF P>65535 THEN GOTO 9978 prevents addressing beyond the 16-bit address space.
• Line 9997 calls USR 8297 to obtain the number of free memory bytes. Address 8297 is a machine code entry point in the ZX81 ROM or a resident utility routine that returns a value usable in BASIC.

Save Routine

Lines 9992–9999 implement a simple save sub-module. If the user presses Enter without typing a filename, S$ is set to "PEEK" by default (line 9995). After saving, the program checks at line 9998 whether S$="PEE%K" — the %K escape denotes an inverse-video K character, making this an exact-match test for the specific auto-saved filename and returning to the help screen in that case; otherwise control passes to GOTO MENU.

Notable Anomalies

• The INKEY$ checks at lines 9983–9987 are sequential single-line tests with no debounce delay. Because the loop iterates very rapidly in SLOW mode, a single keypress may or may not be caught, and multiple tests within one pass of the loop will each see a fresh INKEY$ sample, meaning only the first matching test in sequence will fire per pass.
• Line 9998 compares S$ to "PEE%K" (with an inverse-K) rather than plain "PEEK". This means the post-save branch back to line 9968 will only trigger for that specific encoded string, which would never match a user-typed filename, effectively making the branch dead code unless the default save path sets S$ to that exact internal value.
• Lines 9989–9991 clear the screen, list from line 9967 (displaying the module’s own code), and stop — this is the “return to BASIC” path accessed via key B.

Variable Summary

Variable	Purpose
P	Current memory address being inspected (0–65535)
S$	Filename for SAVE operation; defaults to “PEEK”
MENU	Numeric variable holding the line number of the main program menu