Character Reverse

Have you ever wanted to put a reverse character in a string but you had no way of doing it? Here is a simple answer, a small machine language program that will reverse a character for you:

Decimal          Mnemonic
221,42,8,64      LD IX (nn)
33,70,64         LD HL, nn
35               INC HL
126              LD A (HL)
221,190,0        CP (IX+d)
200              RET cc
254,27           CP n
194,50,64        JP cc, nn
35               INC HL
126              LD A (HL)
198,128          ADD A, n
119              LD (HL) A
195,50,64        JP nn

How It Works

The first line loads the IX register with the memory location of the end of the Basic program, pointed by VARS, then the HL register is loaded, LD HL, nn, with the end of the machine language program. The HL register is then incremented in the line INC HL. In the next line the A register is loaded with the memory contents pointed to by HL. The A register is then compared to the memory location pointed by the IX register. If they are equal, the program returns to Basic. The two will only be equal if it is at the end of the Basic program. The A register is then compared, CP n, with 27, the character number of “.”. If they are not equal, then the program jumps up to INC HL and repeats the cycle. If they are equal, then HL is incremented and the A register is loaded with the memory contents pointed by HL. Then 128 is added, ADD A, n, to A to get the reverse of the character. It is then loaded back into the memory location pointed by HL. Finally the program jumps back to the first INC HL and continues until the end of the Basic program.

Writing the Program

Type the following:

    REM 12345678901234567890123456
100 FOR X=16427 TO 16452
110 PRINT X,,
120 INPUT A
130 IF PEEK(16421)<2 THEN CLS
140 POKE X,A
150 PRINT PEEK(X)
160 NEXT X

Then type in:

10 REM THEN THEN THEN THEN ...

Continue typing THEN until line 1 disappears, then enter:

10 REM DO NOT GO UP

RUN and enter the following values:

16427    221
16428    42
16429    8
16430    64
16431    33
16432    70
16433    64
16434    35
16435    126
16436    221
16437    190
16438    0
16439    200
16440    254
16441    27
16442    194
16443    50
16444    64
16445    35
16446    126
16447    198
16448    128
16449    119
16450    195
16451    50
16452    64

DO NOT PRESS LIST OR HOME. THE PROGRAM WILL BE LOST IF SHOWN.

Delete lines 100-160; then save the program.

To Operate:

Before writing your own program, load this one. Whenever you need the reverse of a character, insert a “.” before the character to be reversed. Then enter GOTO USR(16427). More than one character can be done at a time.

If you wish to use a different character than “.”, POKE 16641, (the character number of your choice).