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Screen Pokes
Volume Number:1
Issue Number:5
Column Tag:BASIC

Professor Mac’s Screen Pokes

By Dave Kelly

Mike Steiner and Michael M. Boy contributed to this article and we acknowledge their valuable con- tributions.

Professor Mac’s Screen Pokes

One day, Professor Mac was poking around in his Mac’s memory when suddenly he fell in and got stuck. Then along came Mike Steiner with screen memory locations. Professor Mac was then able to write his own program to poke his head out of the Mac screen.

The upper left corner of the screen starts at 108288. The locations progress consecutively starting from the upper left corner of the screen to the bottom right corner moving left to right and down the screen. The bottom right corner of the screen is 130175. These locations are for a 128K Mac. Add (512-128)*1024 for a 512K Mac. Be careful to not poke locations before or after the start and end addresses... unpredictable things will happen.

There are 21888 locations mapping the Macintosh screen. Each line (342 lines) of the screen is represented by 64 memory locations. Thus the first line of the screen goes from 108288 to 108351, the second line starts at 108352 and so on.... (remember to add for 512K Mac)

Each memory location contains 1 sixteen bit word which is bit mapped on the Mac screen. From left to right on the screen, the sixteen bits are mapped from hi order bit to low. A typical memory location is shown in figure 1.

The program pokes zeros into the first 18 lines of the screen, hiding the BASIC menu bars. The menus are still active and can be used to trace or stop the program. The image of Professor Mac is read from data statements and poked into the screen memory locations. Most of the time, you probably won’t want to poke the screen directly, there are easier ways to print on the screen. But this method could be used to check status of a particular location of the screen. The FRE(0) command is used to check which Mac (128K or 512K) is being used because the start and ending screen locations are different and have to be adjusted for the 512K Mac.

Thanks and appreciation go to Mike Steiner of Sierra Vista, Az. and Michael M. Boy of Elgin, Az. for submitting the screen locations.

‘Professor Mac’s Screen Poke
‘By Dave Kelly

WINDOW CLOSE 1
start=108288!  ‘Starting address for 128K MAC
ending=130175! ‘Ending address for 128K 

mac512=(512-128)*1024  ‘address offset for 512K MAC
IF FRE(0)>100000! THEN  start=start+mac512: ending=ending+mac512
erase.top.of.screen:
    FOR i= start TO start +64*18
        POKE i,0
    NEXT i
WINDOW  1
TEXTFONT(2):TEXTSIZE(18)
LOCATE 8,10:PRINT “Prof. Mac says: Read MacTutor!”
TEXTSIZE(12):PRINT
PRINT TAB(22);”Hit mouse button to quit”
print.professor.mac:
a=start+64*100 ‘Start at line 100
b=67  ‘print 67 lines 
FOR j=a TO a+b*64 STEP 64
    FOR k= j+30 TO j+36
        READ value
        POKE k,value
    NEXT k
NEXT j
ON MOUSE GOSUB quit
MOUSE ON:RESTORE
GOTO print.professor.mac
quit:
MENU RESET:END
prof.mac.plot.data:
1   DATA &H00,&H00,&H00,&HE0,&H00, &H00,&H00
2   DATA &H00,&H00,&H01,&HF8,&H00, &H00,&H00
3   DATA &H00,&H00,&H07,&HFC,&H00, &H00,&H00
4   DATA &H00,&H00,&H0F,&HFF,&H00, &H00,&H00
5   DATA &H00,&H00,&H3F,&HFF,&HC0, &H00,&H00
6   DATA &H00,&H00,&H7F,&HFF,&HE0, &H00,&H00
7   DATA &H00,&H01,&HFF,&HFF,&HF8, &H00,&H00
8   DATA &H00,&H03,&HFF,&HFF,&HFE, &H00,&H00
9   DATA &H00,&H07,&HFF,&HFF,&HFF, &H80,&H00
10 DATA &H00,&H1F,&HFF,&HFF,&HFF,  &HC0,&H00
11 DATA &H00,&H3F,&HFF,&HFF,&HFF,  &HF0,&H00
12 DATA &H00,&HFF,&HFF,&HFF,&HFF,  &HFC,&H00
13 DATA &H01,&HFF,&HFF,&HF3,&HFF,  &HFE,&H00
14 DATA &H03,&HFF,&HFF,&HE1,&HFF,  &HFF,&H80
15 DATA &H07,&HFF,&HFF,&HE1,&HFF,  &HFF,&HC0
16 DATA &H0F,&HFF,&HFF,&HF2,&H7F,  &HFF,&HC0
17 DATA &H1F,&HFF,&HFF,&HFF,&H9F,  &HFF,&H80
18 DATA &H0F,&HFF,&HFF,&HFF,&HE7,  &HFF,&H00
19 DATA &H07,&HFF,&HFF,&HFF,&HF9,  &HFE,&H00
20 DATA &H01,&HFF,&HFF,&HFF,&HFE,  &H7C,&H00
21 DATA &H03,&HFF,&HFF,&HFF,&HFF,  &H98,&H00
22 DATA &H03,&H3F,&HFF,&HFF,&HFF,  &HE8,&H00
23 DATA &H06,&H0F,&HFF,&HFF,&HFF,  &HFC,&H00
24 DATA &H06,&H07,&HFF,&HFF,&HFF,  &HFF,&HCC
25 DATA &H06,&H01,&HFF,&HFF,&HFF,  &H87,&H00
26 DATA &H06,&H00,&H7F,&HFF,&HFF,  &H0F,&H80
27 DATA &H06,&H00,&H1F,&HFF,&HFF,  &H0F,&H80
28 DATA &H06,&H00,&H0F,&HFF,&HFF,  &H0F,&H80
29 DATA  &H06,&H03,&H83,&HFF,&HFB, &H05,&H00
30 DATA &H06,&H07,&HC0,&HFF,&HF3,  &H00,&H00
31 DATA &H06,&H0F,&H60,&H3F,&HE3,  &H0A,&H80
32 DATA &H06,&H0E,&H30,&H1F,&HC3,  &H05,&H00
33 DATA &H06,&H0C,&H00,&H07,&H83,  &H02,&H00
34 DATA &H06,&H00,&H00,&H01,&H03,  &H08,&H80
35 DATA &H06,&H00,&H00,&H00,&H03,  &H05,&H00
36 DATA &H06,&H00,&H00,&H00,&H03,  &H00,&H00
37 DATA &H06,&H00,&H00,&H00,&H03,  &H0A,&H80
38 DATA &H06,&H80,&H70,&H70,&H0B,  &H05,&H00
39 DATA &H06,&H81,&HF8,&HFC,&H0B,  &H00,&H00
40 DATA &H06,&HC7,&HFD,&HFF,&H1B,  &H0A,&H80
41 DATA &H06,&H7F,&HFF,&HFF,&HF3,  &H00,&H00
42 DATA &H06,&H3F,&HFF,&HFF,&HE3,  &H1F,&H41
43 DATA &H03,&H07,&HFD,&HFF,&H06,  &H04,&H63
44 DATA &H03,&H81,&HF8,&HFB,&H0E,  &H04,&H55
45 DATA &H01,&HC0,&H00,&H00,&H1C,  &H04,&H49
46 DATA &H00,&HE0,&H00,&H00,&H38,  &H04,&H41
47 DATA &H00,&H7F,&HFF,&HFF,&HF0,  &H00,&H00
48 DATA &H00,&H1F,&HFF,&HFF,&HC0,  &H00,&H00
49 DATA &H00,&H08,&H00,&H00,&H40,  &H00,&H00
50 DATA &H00,&H08,&H00,&H00,&H40,  &H00,&H00
51 DATA &H00,&H0F,&HFF,&HFF,&HC0,  &H00,&H00
52 DATA &H00,&H1A,&HAA,&HAA,&HA0,  &H00,&H00
53 DATA &H00,&H35,&H55,&H55,&H50,  &H00,&H00
54 DATA &H00,&H6F,&HEA,&HFE,&HA8,  &H00,&H00
55 DATA &H00,&HD8,&H35,&H83,&H54,  &H00,&H00
56 DATA &H01,&HB0,&H1B,&H01,&HAA,  &H00,&H00
57 DATA &H03,&H50,&H15,&H01,&H55,  &H00,&H00
58 DATA &H06,&HA8,&H2A,&H82,&HAA,  &H80,&H00
59 DATA &H0D,&H57,&HD5,&H7D,&H7F,  &H40,&H00
60 DATA &H1A,&HFA,&HAF,&HAA,&HC1,  &HA0,&H00
61 DATA &H35,&H05,&H50,&H55,&H80,  &HD0,&H00
62 DATA &H6A,&H02,&HA0,&H2A,&H80,  &HA8,&H00
63 DATA &HD6,&H03,&H60,&H35,&H41,  &H54,&H00
64 DATA &HAB,&H06,&HB0,&H6A,&HBE,  &HAC,&H00
65 DATA &HD5,&HFD,&H5F,&HD5,&H55,  &H54,&H00
66 DATA &HAA,&HAA,&HAA,&HAA,&HAA,  &HAC,&H00
67 DATA &HD5,&H55,&H55,&H55,&H55,  &H54,&H00
68 DATA &HFF,&HFF,&HFF,&HFF,&HFF,  &HFC,&H00
 

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