Volume 1 · Number 3 · November 1981

Page 27 of 33

If a valid address is obtained subsequent hexadecimal numbers on the line are entered into memory until the end of the line is reached (detected by means of the nulls with which the screen margins are filled) or until a non-valid entry is found. If all the numbers are valid the modify routine continues on the next line, displaying the updated address and the byte at that address, when further data can be entered. Although the data to be entered is in bytes, the routine which evaluates the successive groups of characters is designed to handle sixteen-bit values, but only the least significant eight bits are put into memory, thus FA, 1FA and 37FA will all go into memory as FA. if the number exceeds FFFF an error message will be generated and the routine will reset to the address at the start of the line, but data will have been entered into memory up to and including the first invalid entry.

If a character is encountered which does not lie in the ranges 0 – 9 or A – F the above error process will normally occur, but here are four exceptions to this. A full stop terminates the Modify command and returns control to the monitor. An oblique stroke changes the address to be modified to the hexadecimal number following the stroke; an error message is produced if the characters following the stroke are not in the ‘hexadecimal’ set, but if no number is entered the address changes to zero, A colon causes the routine to backstep one address. Because the Modify routine leaves the current line when it encounters one of the above three characters, either to return to monitor or to start a new line, you cannot use more than one character per line; you cannot, for example, backstep three spaces be entering 0C90 ::: N/L; only the first will be effective and address 0C8F will be displayed.

The fourth ‘special’ character is the comma; this causes the ASCII code of the following character to be entered into memory. In this case you can enter as many codes as will fit on the line, and you can mix them freely with the usual hexadecimal codes. For example

EF,H,E,L,L,O 00

will be entered as

EF 48 45 4C 4C 4F 00

Note that you do not need to enter spaces to separate the bytes in addition to a comma.


This command resets the addresses of the output and input tables, stored at £0C73 and £0C75, so that output is routed only to the CRT and input is accepted only from the keyboard and serial input port. The U command changes these addresses so that input and output first calls user routines previously specified at £0C7B and £0C78. Thus once the address of a printer routine, for example, has

Page 27 of 33