Flash

Developer(s): Kenneth Majors
Date: 1985
Type: Program
Platform(s): TS 1000
Tags: Banner

Flash is a display program that renders a large ASCII-art banner across the screen using inverse-video block characters to form stylized text. A machine code routine is embedded in the REM statement at line 1 and is called via RAND USR 16514, which points into that REM data. The machine code routine appears to perform a memory operation — iterating over a block, XORing each byte with 0x80 (toggling the high bit, i.e., inverting video attributes or character data), then looping until a counter expires, creating a flashing effect. After the machine code returns, line 60 loops back to line 20 to repeat the flash cycle indefinitely.

Entry for the SyncWare News display program contest.

Program Analysis

Program Structure

The program is compact at six executable lines plus a REM and a banner built from eight PRINT statements. Control flow is straightforward:

  1. Line 1: REM — stores a machine code payload.
  2. Lines 2–8: PRINT AT and PRINT — draw a multi-row inverse-video banner.
  3. Line 9: PRINT AT 18,9 — prints an author credit line.
  4. Line 20: RAND USR VAL "16514" — calls the machine code.
  5. Line 60: GOTO VAL "20" — loops back to re-invoke the machine code, creating continuous animation.

Note that lines 2–9 are only executed on the first pass; the GOTO at line 60 jumps back to line 20, so the PRINT statements are not re-executed on subsequent iterations. This means the banner is drawn once and then the machine code is called in a tight loop.

Machine Code Routine

The REM statement at line 1 contains 29 bytes of Z80 machine code beginning at address 16514 (0x4082), immediately after the REM token and length bytes. The hex sequence is:

OffsetHexZ80 MnemonicComment
02A 0C 40LD HL,(400C)Load HL from address 0x400C
301 19 03LD BC,0x0319Load BC with byte count (793 bytes)
67ELD A,(HL)Load byte at HL into A
7FE 76CP 0x76Compare with HALT / newline token
9CA 91 40JP Z,0x4091Skip (jump past XOR) if byte is 0x76
12EE 80XOR 0x80Toggle bit 7 — invert video flag
1477LD (HL),AWrite back modified byte
1523INC HLAdvance pointer
160BDEC BCDecrement counter
173E 00LD A,0x00Load 0 to test BC
19B8CP BTest B
20C2 88 40JP NZ,0x4088Loop if B ≠ 0
23B9CP CTest C
24C2 88 40JP NZ,0x4088Loop if C ≠ 0
27C9RETReturn to BASIC

The routine loads a base address from memory location 0x400C (which on the ZX81 holds the start of the display file, i.e., DFILE), then iterates over 793 bytes. For each byte, it skips the 0x76 newline token (preserving line structure) and XORs all other bytes with 0x80, toggling the inverse-video bit. Each call to line 20 therefore flips the entire display between normal and inverse, producing a flashing effect. The loop at line 60 keeps this going indefinitely.

Banner Construction

Lines 2–8 use inverse-video characters (shown as % and space in the listing) to draw a large multi-character banner. The PRINT AT 8,0 at line 2 positions the banner in the vertical center of the display. Each row of the banner is a separate PRINT statement, giving seven rows of stylized lettering. Line 9 adds an attribution line at row 18, column 9.

Key BASIC Idioms

  • VAL "16514" in the RAND USR call and VAL "20" in GOTO are memory-saving idioms: storing a number as a string avoids the five-byte floating-point constant the ZX81 would otherwise append to the token.
  • RAND USR is the standard method for calling machine code from BASIC; the return value is discarded.
  • The tight GOTO VAL "20" loop means the screen is never cleared between flash cycles, so the banner persists while only its video polarity alternates.

Notable Techniques

By reading DFILE start from 0x400C rather than using a hardcoded address, the routine is robust against display file relocation (e.g., when the ZX81 is used with a RAM pack). Skipping 0x76 bytes prevents corruption of the display file’s line-end markers, keeping the display structure intact across repeated inversions. The use of separate JP NZ tests for B and C rather than a single DJNZ/DEC BC; JP NZ pair correctly handles the full 16-bit counter.

Potential Anomalies

The JP Z,0x4091 at offset 9 jumps to address 0x4091, which is the INC HL instruction at offset 15 — correctly skipping only the XOR and write-back while still advancing the pointer and decrementing the counter. This is precise and intentional. No bugs are apparent in the logic.

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Source Code

   1 REM 





			
			

		

							

				
					

				

	


	


		
					

				




	
	



	
	
		


			
			
Flash

			
		


	
	
	


							

				





Developer(s): Kenneth Majors


Date: 1985


Type: Program


Platform(s): TS 1000



Tags: Banner



					

				
		

Flash is a display program that renders a large ASCII-art banner across the screen using inverse-video block characters to form stylized text. A machine code routine is embedded in the REM statement at line 1 and is called via RAND USR 16514, which points into that REM data. The machine code routine appears to perform a memory operation — iterating over a block, XORing each byte with 0x80 (toggling the high bit, i.e., inverting video attributes or character data), then looping until a counter expires, creating a flashing effect. After the machine code returns, line 60 loops back to line 20 to repeat the flash cycle indefinitely.





Entry for the SyncWare News display program contest.










Program Analysis





Program Structure





The program is compact at six executable lines plus a REM and a banner built from eight PRINT statements. Control flow is straightforward:





    

  1. Line 1: REM — stores a machine code payload.
    2. 




  2. Lines 2–8: PRINT AT and PRINT — draw a multi-row inverse-video banner.
    3. 




  3. Line 9: PRINT AT 18,9 — prints an author credit line.
    4. 




  4. Line 20: RAND USR VAL "16514" — calls the machine code.
    5. 




  5. Line 60: GOTO VAL "20" — loops back to re-invoke the machine code, creating continuous animation.
    6. 






Note that lines 2–9 are only executed on the first pass; the GOTO at line 60 jumps back to line 20, so the PRINT statements are not re-executed on subsequent iterations. This means the banner is drawn once and then the machine code is called in a tight loop.





Machine Code Routine





The REM statement at line 1 contains 29 bytes of Z80 machine code beginning at address 16514 (0x4082), immediately after the REM token and length bytes. The hex sequence is:





OffsetHexZ80 MnemonicComment
02A 0C 40LD HL,(400C)Load HL from address 0x400C
301 19 03LD BC,0x0319Load BC with byte count (793 bytes)
67ELD A,(HL)Load byte at HL into A
7FE 76CP 0x76Compare with HALT / newline token
9CA 91 40JP Z,0x4091Skip (jump past XOR) if byte is 0x76
12EE 80XOR 0x80Toggle bit 7 — invert video flag
1477LD (HL),AWrite back modified byte
1523INC HLAdvance pointer
160BDEC BCDecrement counter
173E 00LD A,0x00Load 0 to test BC
19B8CP BTest B
20C2 88 40JP NZ,0x4088Loop if B ≠ 0
23B9CP CTest C
24C2 88 40JP NZ,0x4088Loop if C ≠ 0
27C9RETReturn to BASIC





The routine loads a base address from memory location 0x400C (which on the ZX81 holds the start of the display file, i.e., DFILE), then iterates over 793 bytes. For each byte, it skips the 0x76 newline token (preserving line structure) and XORs all other bytes with 0x80, toggling the inverse-video bit. Each call to line 20 therefore flips the entire display between normal and inverse, producing a flashing effect. The loop at line 60 keeps this going indefinitely.





Banner Construction





Lines 2–8 use inverse-video characters (shown as % and space in the listing) to draw a large multi-character banner. The PRINT AT 8,0 at line 2 positions the banner in the vertical center of the display. Each row of the banner is a separate PRINT statement, giving seven rows of stylized lettering. Line 9 adds an attribution line at row 18, column 9.





Key BASIC Idioms





    

  • VAL "16514" in the RAND USR call and VAL "20" in GOTO are memory-saving idioms: storing a number as a string avoids the five-byte floating-point constant the ZX81 would otherwise append to the token.
    • 




  • RAND USR is the standard method for calling machine code from BASIC; the return value is discarded.
    • 




  • The tight GOTO VAL "20" loop means the screen is never cleared between flash cycles, so the banner persists while only its video polarity alternates.
    • 






Notable Techniques





By reading DFILE start from 0x400C rather than using a hardcoded address, the routine is robust against display file relocation (e.g., when the ZX81 is used with a RAM pack). Skipping 0x76 bytes prevents corruption of the display file’s line-end markers, keeping the display structure intact across repeated inversions. The use of separate JP NZ tests for B and C rather than a single DJNZ/DEC BC; JP NZ pair correctly handles the full 16-bit counter.





Potential Anomalies





The JP Z,0x4091 at offset 9 jumps to address 0x4091, which is the INC HL instruction at offset 15 — correctly skipping only the XOR and write-back while still advancing the pointer and decrementing the counter. This is precise and intentional. No bugs are apparent in the logic.


		
							

				












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Source Code





   1 REM \2A\0C\40\01\19\03\7E\FE\76\CA\91\40\EE\80\77\23\0B\3E\00\B8\C2\88\40\B9\C2\88\40\C9\25\1C\1D\1E\1F\20\21\22\23\24\25\1C\1D\1E\1F\20\21\22\23\24\25\1C
   2 PRINT AT VAL "8",VAL "0";"% % % % %  %        %     % % % %  %    %  % "
   3 PRINT "%      %       %  %   %    %  %    %  % "
   4 PRINT "%      %      %    %  %      %    %  % "
   5 PRINT "% % % %   %      %    %   % % %   % % % % %  % "
   6 PRINT "%      %      % % % % %      %  %    %  % "
   7 PRINT "%      %      %    %  %    %  %    % "
   8 PRINT "%      % % % % %  %    %  % % % %   %    %  % "
   9 PRINT AT VAL "18",VAL "9";"BY K.E. MAJORS"
  20 RAND USR VAL "16514"
  60 GOTO VAL "20"









Note: Type-in program listings on this website use ZMAKEBAS notation for graphics characters.





					

				
			




						

				







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Flash

			
		


	
	
	


							

				





Developer(s): Kenneth Majors


Date: 1985


Type: Program


Platform(s): TS 1000



Tags: Banner



					

				
		

Flash is a display program that renders a large ASCII-art banner across the screen using inverse-video block characters to form stylized text. A machine code routine is embedded in the REM statement at line 1 and is called via RAND USR 16514, which points into that REM data. The machine code routine appears to perform a memory operation — iterating over a block, XORing each byte with 0x80 (toggling the high bit, i.e., inverting video attributes or character data), then looping until a counter expires, creating a flashing effect. After the machine code returns, line 60 loops back to line 20 to repeat the flash cycle indefinitely.





Entry for the SyncWare News display program contest.










Program Analysis





Program Structure





The program is compact at six executable lines plus a REM and a banner built from eight PRINT statements. Control flow is straightforward:





    

  1. Line 1: REM — stores a machine code payload.
    2. 




  2. Lines 2–8: PRINT AT and PRINT — draw a multi-row inverse-video banner.
    3. 




  3. Line 9: PRINT AT 18,9 — prints an author credit line.
    4. 




  4. Line 20: RAND USR VAL "16514" — calls the machine code.
    5. 




  5. Line 60: GOTO VAL "20" — loops back to re-invoke the machine code, creating continuous animation.
    6. 






Note that lines 2–9 are only executed on the first pass; the GOTO at line 60 jumps back to line 20, so the PRINT statements are not re-executed on subsequent iterations. This means the banner is drawn once and then the machine code is called in a tight loop.





Machine Code Routine





The REM statement at line 1 contains 29 bytes of Z80 machine code beginning at address 16514 (0x4082), immediately after the REM token and length bytes. The hex sequence is:





OffsetHexZ80 MnemonicComment
02A 0C 40LD HL,(400C)Load HL from address 0x400C
301 19 03LD BC,0x0319Load BC with byte count (793 bytes)
67ELD A,(HL)Load byte at HL into A
7FE 76CP 0x76Compare with HALT / newline token
9CA 91 40JP Z,0x4091Skip (jump past XOR) if byte is 0x76
12EE 80XOR 0x80Toggle bit 7 — invert video flag
1477LD (HL),AWrite back modified byte
1523INC HLAdvance pointer
160BDEC BCDecrement counter
173E 00LD A,0x00Load 0 to test BC
19B8CP BTest B
20C2 88 40JP NZ,0x4088Loop if B ≠ 0
23B9CP CTest C
24C2 88 40JP NZ,0x4088Loop if C ≠ 0
27C9RETReturn to BASIC





The routine loads a base address from memory location 0x400C (which on the ZX81 holds the start of the display file, i.e., DFILE), then iterates over 793 bytes. For each byte, it skips the 0x76 newline token (preserving line structure) and XORs all other bytes with 0x80, toggling the inverse-video bit. Each call to line 20 therefore flips the entire display between normal and inverse, producing a flashing effect. The loop at line 60 keeps this going indefinitely.





Banner Construction





Lines 2–8 use inverse-video characters (shown as % and space in the listing) to draw a large multi-character banner. The PRINT AT 8,0 at line 2 positions the banner in the vertical center of the display. Each row of the banner is a separate PRINT statement, giving seven rows of stylized lettering. Line 9 adds an attribution line at row 18, column 9.





Key BASIC Idioms





    

  • VAL "16514" in the RAND USR call and VAL "20" in GOTO are memory-saving idioms: storing a number as a string avoids the five-byte floating-point constant the ZX81 would otherwise append to the token.
    • 




  • RAND USR is the standard method for calling machine code from BASIC; the return value is discarded.
    • 




  • The tight GOTO VAL "20" loop means the screen is never cleared between flash cycles, so the banner persists while only its video polarity alternates.
    • 






Notable Techniques





By reading DFILE start from 0x400C rather than using a hardcoded address, the routine is robust against display file relocation (e.g., when the ZX81 is used with a RAM pack). Skipping 0x76 bytes prevents corruption of the display file’s line-end markers, keeping the display structure intact across repeated inversions. The use of separate JP NZ tests for B and C rather than a single DJNZ/DEC BC; JP NZ pair correctly handles the full 16-bit counter.





Potential Anomalies





The JP Z,0x4091 at offset 9 jumps to address 0x4091, which is the INC HL instruction at offset 15 — correctly skipping only the XOR and write-back while still advancing the pointer and decrementing the counter. This is precise and intentional. No bugs are apparent in the logic.


		
							

				












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And now, here are a few more words about the last contest. First, we would like to thank all of...













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Source Code





   1 REM \2A\0C\40\01\19\03\7E\FE\76\CA\91\40\EE\80\77\23\0B\3E\00\B8\C2\88\40\B9\C2\88\40\C9\25\1C\1D\1E\1F\20\21\22\23\24\25\1C\1D\1E\1F\20\21\22\23\24\25\1C
   2 PRINT AT VAL "8",VAL "0";"% % % % %  %        %     % % % %  %    %  % "
   3 PRINT "%      %       %  %   %    %  %    %  % "
   4 PRINT "%      %      %    %  %      %    %  % "
   5 PRINT "% % % %   %      %    %   % % %   % % % % %  % "
   6 PRINT "%      %      % % % % %      %  %    %  % "
   7 PRINT "%      %      %    %  %    %  %    % "
   8 PRINT "%      % % % % %  %    %  % % % %   %    %  % "
   9 PRINT AT VAL "18",VAL "9";"BY K.E. MAJORS"
  20 RAND USR VAL "16514"
  60 GOTO VAL "20"









Note: Type-in program listings on this website use ZMAKEBAS notation for graphics characters.





					

				
			




						

				







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	C itemtype='https://schema.org/Blog' itemscope='itemscope' class="wp-singular computer_media-template-default single single-computer_media postid-54960 wp-custom-logo wp-embed-responsive wp-theme-astra wp-child-theme-astra-child ast-desktop ast-separate-container ast-left-sidebar astra-4.12.7 group-blog ast-blog-single-style-1 ast-custom-post-type ast-single-post ast-inherit-site-logo-transparent ast-hfb-header ast-full-width-primary-header ast-box-layout ast-normal-title-enabled astra-addon-4.12.5"E\FE\CA\EE





			
			

		

							

				
					

				

	


	


		
					

				




	
	



	
	
		


			
			
Flash

			
		


	
	
	


							

				





Developer(s): Kenneth Majors


Date: 1985


Type: Program


Platform(s): TS 1000



Tags: Banner



					

				
		

Flash is a display program that renders a large ASCII-art banner across the screen using inverse-video block characters to form stylized text. A machine code routine is embedded in the REM statement at line 1 and is called via RAND USR 16514, which points into that REM data. The machine code routine appears to perform a memory operation — iterating over a block, XORing each byte with 0x80 (toggling the high bit, i.e., inverting video attributes or character data), then looping until a counter expires, creating a flashing effect. After the machine code returns, line 60 loops back to line 20 to repeat the flash cycle indefinitely.





Entry for the SyncWare News display program contest.










Program Analysis





Program Structure





The program is compact at six executable lines plus a REM and a banner built from eight PRINT statements. Control flow is straightforward:





    

  1. Line 1: REM — stores a machine code payload.
    2. 




  2. Lines 2–8: PRINT AT and PRINT — draw a multi-row inverse-video banner.
    3. 




  3. Line 9: PRINT AT 18,9 — prints an author credit line.
    4. 




  4. Line 20: RAND USR VAL "16514" — calls the machine code.
    5. 




  5. Line 60: GOTO VAL "20" — loops back to re-invoke the machine code, creating continuous animation.
    6. 






Note that lines 2–9 are only executed on the first pass; the GOTO at line 60 jumps back to line 20, so the PRINT statements are not re-executed on subsequent iterations. This means the banner is drawn once and then the machine code is called in a tight loop.





Machine Code Routine





The REM statement at line 1 contains 29 bytes of Z80 machine code beginning at address 16514 (0x4082), immediately after the REM token and length bytes. The hex sequence is:





OffsetHexZ80 MnemonicComment
02A 0C 40LD HL,(400C)Load HL from address 0x400C
301 19 03LD BC,0x0319Load BC with byte count (793 bytes)
67ELD A,(HL)Load byte at HL into A
7FE 76CP 0x76Compare with HALT / newline token
9CA 91 40JP Z,0x4091Skip (jump past XOR) if byte is 0x76
12EE 80XOR 0x80Toggle bit 7 — invert video flag
1477LD (HL),AWrite back modified byte
1523INC HLAdvance pointer
160BDEC BCDecrement counter
173E 00LD A,0x00Load 0 to test BC
19B8CP BTest B
20C2 88 40JP NZ,0x4088Loop if B ≠ 0
23B9CP CTest C
24C2 88 40JP NZ,0x4088Loop if C ≠ 0
27C9RETReturn to BASIC





The routine loads a base address from memory location 0x400C (which on the ZX81 holds the start of the display file, i.e., DFILE), then iterates over 793 bytes. For each byte, it skips the 0x76 newline token (preserving line structure) and XORs all other bytes with 0x80, toggling the inverse-video bit. Each call to line 20 therefore flips the entire display between normal and inverse, producing a flashing effect. The loop at line 60 keeps this going indefinitely.





Banner Construction





Lines 2–8 use inverse-video characters (shown as % and space in the listing) to draw a large multi-character banner. The PRINT AT 8,0 at line 2 positions the banner in the vertical center of the display. Each row of the banner is a separate PRINT statement, giving seven rows of stylized lettering. Line 9 adds an attribution line at row 18, column 9.





Key BASIC Idioms





    

  • VAL "16514" in the RAND USR call and VAL "20" in GOTO are memory-saving idioms: storing a number as a string avoids the five-byte floating-point constant the ZX81 would otherwise append to the token.
    • 




  • RAND USR is the standard method for calling machine code from BASIC; the return value is discarded.
    • 




  • The tight GOTO VAL "20" loop means the screen is never cleared between flash cycles, so the banner persists while only its video polarity alternates.
    • 






Notable Techniques





By reading DFILE start from 0x400C rather than using a hardcoded address, the routine is robust against display file relocation (e.g., when the ZX81 is used with a RAM pack). Skipping 0x76 bytes prevents corruption of the display file’s line-end markers, keeping the display structure intact across repeated inversions. The use of separate JP NZ tests for B and C rather than a single DJNZ/DEC BC; JP NZ pair correctly handles the full 16-bit counter.





Potential Anomalies





The JP Z,0x4091 at offset 9 jumps to address 0x4091, which is the INC HL instruction at offset 15 — correctly skipping only the XOR and write-back while still advancing the pointer and decrementing the counter. This is precise and intentional. No bugs are apparent in the logic.


		
							

				












Content















Appears On















Related Products















Related Articles





And now, here are a few more words about the last contest. First, we would like to thank all of...













Related Content















Image Gallery











Source Code





   1 REM \2A\0C\40\01\19\03\7E\FE\76\CA\91\40\EE\80\77\23\0B\3E\00\B8\C2\88\40\B9\C2\88\40\C9\25\1C\1D\1E\1F\20\21\22\23\24\25\1C\1D\1E\1F\20\21\22\23\24\25\1C
   2 PRINT AT VAL "8",VAL "0";"% % % % %  %        %     % % % %  %    %  % "
   3 PRINT "%      %       %  %   %    %  %    %  % "
   4 PRINT "%      %      %    %  %      %    %  % "
   5 PRINT "% % % %   %      %    %   % % %   % % % % %  % "
   6 PRINT "%      %      % % % % %      %  %    %  % "
   7 PRINT "%      %      %    %  %    %  %    % "
   8 PRINT "%      % % % % %  %    %  % % % %   %    %  % "
   9 PRINT AT VAL "18",VAL "9";"BY K.E. MAJORS"
  20 RAND USR VAL "16514"
  60 GOTO VAL "20"









Note: Type-in program listings on this website use ZMAKEBAS notation for graphics characters.





					

				
			




						

				







Tags





Archived Media  Downloadable TS 1000 Banner 








					

				



			

			
		
	



	
 
	


	












	
                
                	Scroll to Top


			
			












	BE





			
			

		

							

				
					

				

	


	


		
					

				




	
	



	
	
		


			
			
Flash

			
		


	
	
	


							

				





Developer(s): Kenneth Majors


Date: 1985


Type: Program


Platform(s): TS 1000



Tags: Banner



					

				
		

Flash is a display program that renders a large ASCII-art banner across the screen using inverse-video block characters to form stylized text. A machine code routine is embedded in the REM statement at line 1 and is called via RAND USR 16514, which points into that REM data. The machine code routine appears to perform a memory operation — iterating over a block, XORing each byte with 0x80 (toggling the high bit, i.e., inverting video attributes or character data), then looping until a counter expires, creating a flashing effect. After the machine code returns, line 60 loops back to line 20 to repeat the flash cycle indefinitely.





Entry for the SyncWare News display program contest.










Program Analysis





Program Structure





The program is compact at six executable lines plus a REM and a banner built from eight PRINT statements. Control flow is straightforward:





    

  1. Line 1: REM — stores a machine code payload.
    2. 




  2. Lines 2–8: PRINT AT and PRINT — draw a multi-row inverse-video banner.
    3. 




  3. Line 9: PRINT AT 18,9 — prints an author credit line.
    4. 




  4. Line 20: RAND USR VAL "16514" — calls the machine code.
    5. 




  5. Line 60: GOTO VAL "20" — loops back to re-invoke the machine code, creating continuous animation.
    6. 






Note that lines 2–9 are only executed on the first pass; the GOTO at line 60 jumps back to line 20, so the PRINT statements are not re-executed on subsequent iterations. This means the banner is drawn once and then the machine code is called in a tight loop.





Machine Code Routine





The REM statement at line 1 contains 29 bytes of Z80 machine code beginning at address 16514 (0x4082), immediately after the REM token and length bytes. The hex sequence is:





OffsetHexZ80 MnemonicComment
02A 0C 40LD HL,(400C)Load HL from address 0x400C
301 19 03LD BC,0x0319Load BC with byte count (793 bytes)
67ELD A,(HL)Load byte at HL into A
7FE 76CP 0x76Compare with HALT / newline token
9CA 91 40JP Z,0x4091Skip (jump past XOR) if byte is 0x76
12EE 80XOR 0x80Toggle bit 7 — invert video flag
1477LD (HL),AWrite back modified byte
1523INC HLAdvance pointer
160BDEC BCDecrement counter
173E 00LD A,0x00Load 0 to test BC
19B8CP BTest B
20C2 88 40JP NZ,0x4088Loop if B ≠ 0
23B9CP CTest C
24C2 88 40JP NZ,0x4088Loop if C ≠ 0
27C9RETReturn to BASIC





The routine loads a base address from memory location 0x400C (which on the ZX81 holds the start of the display file, i.e., DFILE), then iterates over 793 bytes. For each byte, it skips the 0x76 newline token (preserving line structure) and XORs all other bytes with 0x80, toggling the inverse-video bit. Each call to line 20 therefore flips the entire display between normal and inverse, producing a flashing effect. The loop at line 60 keeps this going indefinitely.





Banner Construction





Lines 2–8 use inverse-video characters (shown as % and space in the listing) to draw a large multi-character banner. The PRINT AT 8,0 at line 2 positions the banner in the vertical center of the display. Each row of the banner is a separate PRINT statement, giving seven rows of stylized lettering. Line 9 adds an attribution line at row 18, column 9.





Key BASIC Idioms





    

  • VAL "16514" in the RAND USR call and VAL "20" in GOTO are memory-saving idioms: storing a number as a string avoids the five-byte floating-point constant the ZX81 would otherwise append to the token.
    • 




  • RAND USR is the standard method for calling machine code from BASIC; the return value is discarded.
    • 




  • The tight GOTO VAL "20" loop means the screen is never cleared between flash cycles, so the banner persists while only its video polarity alternates.
    • 






Notable Techniques





By reading DFILE start from 0x400C rather than using a hardcoded address, the routine is robust against display file relocation (e.g., when the ZX81 is used with a RAM pack). Skipping 0x76 bytes prevents corruption of the display file’s line-end markers, keeping the display structure intact across repeated inversions. The use of separate JP NZ tests for B and C rather than a single DJNZ/DEC BC; JP NZ pair correctly handles the full 16-bit counter.





Potential Anomalies





The JP Z,0x4091 at offset 9 jumps to address 0x4091, which is the INC HL instruction at offset 15 — correctly skipping only the XOR and write-back while still advancing the pointer and decrementing the counter. This is precise and intentional. No bugs are apparent in the logic.


		
							

				












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   1 REM \2A\0C\40\01\19\03\7E\FE\76\CA\91\40\EE\80\77\23\0B\3E\00\B8\C2\88\40\B9\C2\88\40\C9\25\1C\1D\1E\1F\20\21\22\23\24\25\1C\1D\1E\1F\20\21\22\23\24\25\1C
   2 PRINT AT VAL "8",VAL "0";"% % % % %  %        %     % % % %  %    %  % "
   3 PRINT "%      %       %  %   %    %  %    %  % "
   4 PRINT "%      %      %    %  %      %    %  % "
   5 PRINT "% % % %   %      %    %   % % %   % % % % %  % "
   6 PRINT "%      %      % % % % %      %  %    %  % "
   7 PRINT "%      %      %    %  %    %  %    % "
   8 PRINT "%      % % % % %  %    %  % % % %   %    %  % "
   9 PRINT AT VAL "18",VAL "9";"BY K.E. MAJORS"
  20 RAND USR VAL "16514"
  60 GOTO VAL "20"









Note: Type-in program listings on this website use ZMAKEBAS notation for graphics characters.





					

				
			




						

				







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	\B8\C2\B9\C2\C9 itemtype='https://schema.org/Blog' itemscope='itemscope' class="wp-singular computer_media-template-default single single-computer_media postid-54960 wp-custom-logo wp-embed-responsive wp-theme-astra wp-child-theme-astra-child ast-desktop ast-separate-container ast-left-sidebar astra-4.12.7 group-blog ast-blog-single-style-1 ast-custom-post-type ast-single-post ast-inherit-site-logo-transparent ast-hfb-header ast-full-width-primary-header ast-box-layout ast-normal-title-enabled astra-addon-4.12.5"C itemtype='https://schema.org/Blog' itemscope='itemscope' class="wp-singular computer_media-template-default single single-computer_media postid-54960 wp-custom-logo wp-embed-responsive wp-theme-astra wp-child-theme-astra-child ast-desktop ast-separate-container ast-left-sidebar astra-4.12.7 group-blog ast-blog-single-style-1 ast-custom-post-type ast-single-post ast-inherit-site-logo-transparent ast-hfb-header ast-full-width-primary-header ast-box-layout ast-normal-title-enabled astra-addon-4.12.5"D itemtype='https://schema.org/Blog' itemscope='itemscope' class="wp-singular computer_media-template-default single single-computer_media postid-54960 wp-custom-logo wp-embed-responsive wp-theme-astra wp-child-theme-astra-child ast-desktop ast-separate-container ast-left-sidebar astra-4.12.7 group-blog ast-blog-single-style-1 ast-custom-post-type ast-single-post ast-inherit-site-logo-transparent ast-hfb-header ast-full-width-primary-header ast-box-layout ast-normal-title-enabled astra-addon-4.12.5"E itemtype='https://schema.org/Blog' itemscope='itemscope' class="wp-singular computer_media-template-default single single-computer_media postid-54960 wp-custom-logo wp-embed-responsive wp-theme-astra wp-child-theme-astra-child ast-desktop ast-separate-container ast-left-sidebar astra-4.12.7 group-blog ast-blog-single-style-1 ast-custom-post-type ast-single-post ast-inherit-site-logo-transparent ast-hfb-header ast-full-width-primary-header ast-box-layout ast-normal-title-enabled astra-addon-4.12.5"F itemtype='https://schema.org/Blog' itemscope='itemscope' class="wp-singular computer_media-template-default single single-computer_media postid-54960 wp-custom-logo wp-embed-responsive wp-theme-astra wp-child-theme-astra-child ast-desktop ast-separate-container ast-left-sidebar astra-4.12.7 group-blog ast-blog-single-style-1 ast-custom-post-type ast-single-post ast-inherit-site-logo-transparent ast-hfb-header ast-full-width-primary-header ast-box-layout ast-normal-title-enabled astra-addon-4.12.5"C itemtype='https://schema.org/Blog' itemscope='itemscope' class="wp-singular computer_media-template-default single single-computer_media postid-54960 wp-custom-logo wp-embed-responsive wp-theme-astra wp-child-theme-astra-child ast-desktop ast-separate-container ast-left-sidebar astra-4.12.7 group-blog ast-blog-single-style-1 ast-custom-post-type ast-single-post ast-inherit-site-logo-transparent ast-hfb-header ast-full-width-primary-header ast-box-layout ast-normal-title-enabled astra-addon-4.12.5"D itemtype='https://schema.org/Blog' itemscope='itemscope' class="wp-singular computer_media-template-default single single-computer_media postid-54960 wp-custom-logo wp-embed-responsive wp-theme-astra wp-child-theme-astra-child ast-desktop ast-separate-container ast-left-sidebar astra-4.12.7 group-blog ast-blog-single-style-1 ast-custom-post-type ast-single-post ast-inherit-site-logo-transparent ast-hfb-header ast-full-width-primary-header ast-box-layout ast-normal-title-enabled astra-addon-4.12.5"E itemtype='https://schema.org/Blog' itemscope='itemscope' class="wp-singular computer_media-template-default single single-computer_media postid-54960 wp-custom-logo wp-embed-responsive wp-theme-astra wp-child-theme-astra-child ast-desktop ast-separate-container ast-left-sidebar astra-4.12.7 group-blog ast-blog-single-style-1 ast-custom-post-type ast-single-post ast-inherit-site-logo-transparent ast-hfb-header ast-full-width-primary-header ast-box-layout ast-normal-title-enabled astra-addon-4.12.5"F itemtype='https://schema.org/Blog' itemscope='itemscope' class="wp-singular computer_media-template-default single single-computer_media postid-54960 wp-custom-logo wp-embed-responsive wp-theme-astra wp-child-theme-astra-child ast-desktop ast-separate-container ast-left-sidebar astra-4.12.7 group-blog ast-blog-single-style-1 ast-custom-post-type ast-single-post ast-inherit-site-logo-transparent ast-hfb-header ast-full-width-primary-header ast-box-layout ast-normal-title-enabled astra-addon-4.12.5"C
   2 PRINT AT VAL "8",VAL "0";"% % % % %  %        %     % % % %  %    %  % "
   3 PRINT "%      %       %  %   %    %  %    %  % "
   4 PRINT "%      %      %    %  %      %    %  % "
   5 PRINT "% % % %   %      %    %   % % %   % % % % %  % "
   6 PRINT "%      %      % % % % %      %  %    %  % "
   7 PRINT "%      %      %    %  %    %  %    % "
   8 PRINT "%      % % % % %  %    %  % % % %   %    %  % "
   9 PRINT AT VAL "18",VAL "9";"BY K.E. MAJORS"
  20 RAND USR VAL "16514"
  60 GOTO VAL "20"

Note: Type-in program listings on this website use ZMAKEBAS notation for graphics characters.

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