Volume 1, Number 2 – September 1981

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enter Wxxxx yyyy to write the memory from xxxx to yyyy to tape, you can obtain a second copy of the same region merely by entering W, because the original parameters are still in the workspace, providing that you haven’t pressed RESET. Similarly, if you set the number of lines to be displayed in the tabulate command, this value is retained until re-entered, or set to zero by RESET.



This command gives the sum and difference of the two sixteen-bit values, together with the ‘offset’ for use in relative jump instructions. For example, to perform a relative jump from £0C80 to an instruction at £0CA0 the offset necessary is given by entering AC80 CA0, when the monitor will respond with 1920 0020 1E. Thus the necessary relative jump is:

£0C80  18 1E     JR £0CA0

Note that the address of the relative jump instruction is entered, not the address of the offset byte. If the offset lies outside the permissible range for relative jumps two question marks are printed in place of the offset.


The use of the breakpoint command, Bxxxx, to insert restart £E7 at address xxxx for the debugging of programs was covered in the first article in this series. The breakpoint function is turned off by entering B0. Nas-Sys 1 assumes that there is ROM at address £0000, so it goes ahead and ‘ inserts’ £E7 at this address; of course, this has no effect because you cannot change the ROM by writing to it, so the breakpoint is inoperative. In Nas-Sys 3 no attempt is made to insert £E7, or to replace the original code at the breakpoint, if the breakpoint address is zero. This means the breakpoint routine can be used if you have RAM at address 0000, i.e., in disc systems.

COPY Cxxxx yyyy zzzz

The command copies a block of data zzzz bytes long from address xxxx to address yyyy. For general copying of data it is best to use the intelligent copy command, I, which thinks about what it is doing to ensure that data is not overwritten (see below). In fact the C command is most useful for filling a block of memory with a specific byte. Thus if you modify £1000 to £00, and then execute C1000 1001 400 you will fill the 1K block starting at £1000 with zeros. Because I never use the Copy routine I have replaced it with a command to compare two blocks of memory, for which a listing was given in the first article in the series.

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