Ireg 1000

The TS1000 owners manual states (p. 123) that upon return from machine code, the I register must have the value 1Eh. However, it can be a lot of fun breaking the rules. The I register points to the upper byte of the character generator table in the ROM. If we change it, the character set changes to a random hash, which can be a useful effect.

This program works by setting a basic variable, IREG, equal to what we want in the I register. RAND USR 16514 will the change the value in the I register. When this register is changed, the screen turns to chaos, which is great for getting peoples attention and simulating explosions.

Only the 1 REM line is necessary for the program to operate, so after you enter and test the program, you may delete all of the other lines. The REM line must contain at least 38 spaces. Use your favorite POKEr program to enter the data. To make this routine relocatable, you would have to change the first instruction (LD HL, 40A3) to point to the name of the variable that you would like to look up the value of(IREG in this case). If you would like to change the name of the variable, then you can change the data at address 40A3 to any variable name that you desire. Just end the name with a + sign.

The Basic program listing has several REM lines containing the source listing of the machine code. It is not necessary to enter these lines into the program.

Enter the data left to right and top to bottom.

DECIMAL POKE TABLE

 33 163  64  34  22  64  14   0
205  28  17  48   2 207   1  35
237  91  28  64 205 246  25 235
 34  28  64 205 205  21 237  71
201  46  55  42  44  21

IREG Basic Listing

   1 REM 5▒RND6-RND: LN 0)K▒INT
▒7 GOSUB ?0RNDLN PLOT ; FOR 60RN
DLN LN + GOSUB ?TAN IREG+
 110 REM LD HL,/1000
 120 REM LD (H4016),HL
 130 REM LD C,0
 140 REM CALL H111C
 150 REM JR NC,/180
 160 REM RST 08
 170 REM DATA H01
 180 REM INC HL
 220 REM LD DE,(H401C)
 230 REM CALL H19F6
 240 REM EX DE,HL
 250 REM LD (H401C),HL
 260 REM CALL H15CD
 270 REM LD I,A
 280 REM RET
1000 REM DATA "IREG+"
2000 REM END
4000 LIST 110
5000 FOR J=0 TO 30 STEP 2
5010 LET IREG=J
5020 RAND USR 16514
5025 NEXT J

IREG Disassembly

4082 21A340     LD HL,40A3
4085 221640     LD (CHADD),HL
4088 0E00       LD C,00
408A CD1C11     CALL 111C
408D 3002       JR NC 4091
408F CF         RST 08H
4090 01          ERROR 2
4091 23         INC HL
4092 ED5B1C40   LD DE,(STKND)
4096 CDF619     CALL 19F6
4099 EB         EX DE,HL
409A 221C40     LD (STKND),HL
409D CDCD15     CALL 15CD
40A0 ED47       LD I,A
40A2 C9         RET
40A3 2E37       LD L,37
40A5 2A2C15     LD HL,(152C)
40A8 76         HLT
40A9 00         NOP
40AA 6E         LD L,(HL)
40AB 0D         DEC C
40AC 00         NOP