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Following the instruction of Chapter 10 of First Steps in Z80 Assembly Language by Darryl Sloan: Firstly, we fill the screen with a BASIC program of random colored "hello" messages. The assembler code, as I understand it, is then able to copy that screen data and print it back after the screen is cleared.

However, I just get a black screen instead of the random collection of coloured hello.

This is the asm

ORG 50000; Origin statement
LD HL, 30000 ; start address
LD BC, 6912 ; number of bytes to copy
LD DE, 16384 ; destination address
LDIR
RET

and the BASIC

40 PRINT INK INT (RND*8); PAPER INT (RND*8); "Hello";
50 GO TO 50

I understand the concept and the code, however unlike the author, I'm not using an emulator with a built in assembler. - Which is where I may be going wrong.

  • Write assembler code in notepad.
  • Convert asm code to tap
  • Load in tap file
  • Type the two lines of BASIC in the emulator to the existing code
  • RUN program in emulator
  • RANDOMIZE USR 50000

The source document is made free available and can be found here

Ghoul Fool
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    As far as I understand the assembly code is copying stuff from address 30000 ... so you need to put something there first. – Jester Nov 13 '21 at 16:38
  • Are you able to provide a link to some legal online copy of said chapter? -- Anyway, the assembler program just copies, there is no erasing, and no restoring. Worst, the destination address 16384 is the start of the RAM, and the copy will overwrite the system variables, the BASIC program, and a lot more that follows... – the busybee Nov 13 '21 at 19:04
  • @thebusybee I've added a link to the chapter. Two lines of the code get changed to change the effect - which could explain a few thing. I've also tried recreating in using ZXSPin (with built in assembler) but the code doesn't appear to do anything. I may be doing something out of order. – Ghoul Fool Nov 14 '21 at 10:53

1 Answers1

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As @Jester already pointed out, that snippet of code you posted is copying the contents of memory 30000-36911 to the display memory, which at the time of execution is most likely filled with 0s, hence the black screen.

You need to transfer the contents of the screen to that area of memory first with a complementary transfer routine:

ORG 50020; Origin statement
LD HL, 16384; start address of screen
LD BC, 6912 ; number of bytes to copy
LD DE, 30000; destination address
LDIR
RET

And invoke from basic as usual:

RANDOMIZE USR 50020 --> save screen to ram

RANDOMIZE USR 50000 --> transfer ram back to screen

MutenFuzz
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