Use you ROM



          Norman Green in Derbyshire makes life a

         little easier for 16 and 48K programmers



When I was a schoolboy, I was often told to use my loaf.

Any day now, I expect to hear some youngster told to use

his ROM! It would be sound advice, even though not a true

parallel. Using one's loaf is to use one's own brains: to

use your ROM would be to make use of someone else's; and

if, like me, your computer is a Spectrum, the brains you

would be using are those of Uncle Clive and his team of

experts at Sinclair Research. The Spectrum ROM, with over

sixteen thousand bytes of permanent memory, contains many

very useful subroutines just waiting to be called by some

grateful user (or should I say USR?).

   Although the ROM can be called from the keyboard using

the USR key in instructions such as 'RANDOMIZE USR 0000' or

'LET l=USR 0000', the ROM routines are at their most useful

when called from machine code programs, in which the pro-

grammer can easily arrange for the entry conditions of any

particular routine to be satisfied.

   In this short article, I shall limit myself to discus-

sing just two of the many routines in the Spectrum ROM and

then show how they may be linked together to provide a

short but very powerful utility program. Readers who do not

yet feel at home with machine code will nevertheless be

able to key in and use the concluding program, which is

written entirely in Basic with the machine code stored in a

data line from which it will be transferred to a temporary

home in the printer buffer section of RAM each time the

program is RUN.



 Into the ROM



The first of the aforementioned routines is situated in the

Spectrum ROM at decimal address 6510. Its function is to

determine the address in RAM of the first byte in a program

line whose line number has been previously entered into the

HL register pair. After calling 6510, the HL register will

contain the required address.

   The other routine may be entered at 6629. Its function

is to delete lines from the Basic program. [Or variables,

or, less usefully for the normal programmer, channel data

or any temporary data stored between the system variables

and the end of the calculator stack. It will adjust pointer

system variables (such as PROG) as necessary.] Before this

routine is called, the address of the first byte in the

lowest-numbered line of the block to be deleted must be

loaded into the DE register pair, and the address of the

byte following the last byte in the highest-numbered line

of the block must be loaded into the HL register pair.

   Although they may sound complicated, use of these two

routines is simplicity itself. I have seen complicated

machine code programs of considerable length which take

several minutes to delete a block of lines. The code which

I am about to offer is a mere 19 bytes long and can delete

a large chunk of unwanted program in a fraction of a

second. It is more straightforward than the much-used

method of corrupting the length-of-line bytes in order to

fool the computer into thinking a block of lines is all one

single line, and then having to delete this by keying.

   For those familiar with assembly language mnemonics

these are printed below, alongside the 19 bytes of decimal

machine code.

   Any line numbers are admissible, providing the first is

not higher than the second. If the two are equal, then

single line deletion is effected. If numbers which have not

been allocated to program lines are entered, then all lines

encompassed by the two numbers will be deleted. (This also

applies to the number zero and to all numbers greater than

9999).

   I conclude with the promised Basic program. The lines

are numbered 9990 to 9998 so that if transferred to tape by

keying SAVE "Deletions", it may be recalled whenever

required by keying MERGE "Deletions", when the high line

numbers are unlikely to clash with the program being

edited. When no longer required, "Deletions" may be used to

delete itself; although it should be noted that the dele-

tion of line 9998 by this method will produce the report

"C Nonsense in BASIC, 9998:1", but on this occasion it may

be ignored.

___________________________________________________________



 Ld HL, 0      33, 0*, 0*,    First line no. into HL.

 Call 6510    205,110, 25,    Using your ROM.

 Push HL      229,            Store returned address on

                              stack.

 Ld HL, 0      33, 0*, 0*,    Second line no. into HL.

 Inc HL        35,            Line no. above deletion

                              block.

 Call 6510    205,110, 25,    Using your ROM.

 Pop DE       209,            Reclaim address from stack

                              to DE.

 Call 6629    205,229, 25,    Using your ROM.

 Ret          201.            Return to Basic.



 * * Line numbers are poked here in the usual way,

     least significant byte first.



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