I/O Ports on the Models 3 and 4
The Models 3 and 4 used I/O ports extensively for their dealings with hardware. Even though
the keyboard and display were memory mapped, as in the Model 1, almost all the other hardware
of the machine was accessed through port I/O. The exception was that the Model 3 printer could
be accessed at the same memory location as on the Model 1.
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Miscellaneous Control ports
84H Mod IV - various controls.
Input is reserved.
Output:
Bit 0: Video memory, Keyboard memory, and Model III ROM.
(See table 1)
Bit 1: Same as bit 0. (See Table 1)
Bit 2: Video display mode. 0 = 64 by 16, 1 = 80 by 24.
Bit 3: Reverse Video.
Bit 4: Ram bank select. (See table 2)
Bit 5: Ram bank select. (See table 2)
Bit 6: Ram bank select. (See table 2)
Bit 7: Video page select (64 x 16 mode) 0=page 0, 1=page 1.
Table 1:
Bits Model III Video and
1 0 ROMs Enabled. Keyboard status.
- - -------------- ----------------
0 0 Yes Model III.
0 1 No Model III.
1 0 No Model 4. (In)
1 1 No Model 4. (Out)
Table 2:
Bits Lower 32k Upper 32K
6 5 4 RAM RAM
- - - --------- ---------
0 0 0 Bank 0 Bank 1
0 1 0 Bank 0 Bank 2
0 1 1 Bank 0 Bank 3
1 1 0 Bank 2 Bank 1
1 1 1 Bank 3 Bank 1
85H - 87H. Same as 84H.
E0H Maskable interupt
E4H Input: Read NMI status
Output: Select NMI options
ECH Input: Reset clock
Output: Various controls/
Bit 7 not used
Bit 6 CPU clock speed 0 = 2 mhz, 1 = 4 mhz (Mod IV only)
Bit 5 Video waits 0 = disable, 1 = enable
Bit 4 I/O bus 0 = disable, 1 = enable
Bit 3 Alt. char. 0 = disable, 1 = enable
Bit 2 Double width 0 = normal, 1 = double
Bit 1 Cass motor 0 = on, 1 = off
Bit 0 not used
F8H Line printer address port.
Input: 80H Busy.
40H Out of paper
20H Unit select
10H Fault
Output: Data sent to the printer with a strobe
FFH Cassette port
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Model 3/4 I/O Ports