Radio Shack TRS-80 Model 4 Technical Reference Manual

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Following is a Bit Map of the appropriate ports in the Model 4. Note that this is an “internal” bit map only. For bit
maps of the optional devices, refer to the appropriate section of the desired manual.
Model 4 Port Bit Map
Port D7 D6 D5 D4 D3 D2 D1 D0
FC-FF Cass Cassette
(READ) Data
500 bd
(MIRROR of PORT EC) Data
1500 bd
FC - FF (Note, also resets cassette data latch) cass. cassette
(WRITE)xxxxxx outdata out
F8 - FB Prntr Prntr Prntr Prntr x x x x
(READ) BUSY Paper Select Fault x x x x
F8 - FB Prntr Prntr Prntr Prntr Prntr Prntr Prntr Prntr
(WRITE)D7D6D5D4D3D2D1D0
EC - EF (Any Read causes reset of Real Time Clock Interrupt)
EC - EF x CPU x Enable Enable Mode Cass x
(WRITE) x Fast x EX I/O Altset Select Mot On x
E0 - E3 x Receive Receive Xmit 10 Bus RTC C Fall C Rise
(READ) x Error Data Empty Int Int Int Int
E0 - E3 x Enable Enable Enable Enable Enable Enable Enable
(WRITE) x Rec Err Rec Data Xmit Emp 10 Int RT Int CF Int CR Int
90-93xxxxxx xSound
(WRITE)xxxxxx xBit
84 - 87 Page Fix Upr Memory Memory Invert 80/64 Select Select
(WRITE) Memory Bit 1 Bit 0 Video Bit 1 Bit 0
Memory mapping is accomplished by PAL U59 in the Basic 16K or 64K computer. In a 128K system, PAL U72,
along with the select and memory bits of the options register, also enter into the memory mapping function.
Four memory maps are listed below. Memory Map I is compatible with the Model III. Note that there are two 32K
banks in the 64K system, which can be interchanged with either position of the upper two banks of a 128K system.
The 128K system has four moveable 32K banks. Also note, in the Model III mode, that decoding for the printer
status read (37E8 and 37E9 hexadecimal) is accomplished by U93 and leftover gates from U40, U46, U51, U54,
U60, and U62.
Memory Map I - Model III Mode
0000 - 1FFF
ROM A (8K)
2000 - 2FFF
ROM B (4K)
3000 - 37FF
ROM C (2K) - Less 37E8 - 37E9
37E8 - 37E9
Printer Status Port
3800 - 38FF
Keyboard
3C00 - 3FFF
Video RAM (Page bit selects 1K of 2K)
*
4000 - 7FFF
RAM (l6Ksystem)
*
4000 - FFFF
RAM (64K system)
Memory Map II
0000 - 37FF
RAM (14K)
3800 - 38FF
Keyboard

Radio Shack TRS-80 Model 4 Specifications

General

Brand	Radio Shack
Model	TRS-80 Model 4
Category	Desktop
Language	English