Fig. 2 - Memory configuration and selection circuit - block diagram
Its operation makes use of the fact that the
R
register is 7-bit
incremented after each instruction fetch cycle and appears entirely (8-bit) on
the address bus lines
A0
—
A7
, the moment of its appearance being signaled by
the
RFSH
signal.
A change of bit 7 of the
R
register by the instruction
LD R,A
is shown on
the address bus (
BA7
) only after the next instruction code was already fetched.
This instruction can be a single-byte jump instruction, such as
RST n
or
JP (HL)
, which can achieve the jump of the program counter to any address
within the 64 KB memory space. The memory configuration switching
sequence:
LD R,A ; JP (HL)
is detailed in
Fig.3
.
The two flip-flops (
U36
) are forced to “1” at power-up, by a circuit made
of
C15
,
D02
,
R09
,
D03
.
9
Summary of Contents for CoBra
Page 20: ...Fig 10 Keyboard schematic 20 ...
Page 21: ...Fig 11 Keyboard schematic 21 ...
Page 23: ...Fig 13 Power source schematic 23 ...
Page 39: ...9 APPENDIX 1 CoBra Microcomputer Schematics 39 ...
Page 40: ...Fig A1 1 CoBra Microcomputer Central Processing Unit 40 ...
Page 41: ...Fig A1 2 CoBra Microcomputer Configurator and Selector Circuit 41 ...
Page 42: ...Fig A1 3 CoBra Microcomputer Read Only Memory Circuit 42 ...
Page 43: ...Fig A1 4 CoBra Microcomputer DRAM Memory Circuit 43 ...
Page 44: ...Fig A1 5 CoBra Microcomputer Memory Access Prioritizer and Command Logic 44 ...
Page 45: ...Fig A1 6 CoBra Microcomputer Video Address Generator Circuit 45 ...
Page 46: ...Fig A1 7 CoBra Microcomputer Video Address Multiplexer Circuit 46 ...
Page 47: ...Fig A1 8 CoBra Microcomputer Video Memory Circuit 47 ...
Page 48: ...Fig A1 9 CoBra Microcomputer Video Sync Pulses Generator Circuit 48 ...
Page 49: ...Fig A1 10 CoBra Microcomputer Video Signal Shape Generator 49 ...
Page 50: ...Fig A1 11 CoBra Microcomputer Interfaces 50 ...
Page 51: ...Fig A1 12 CoBra Microcomputer Voltage Level Adapter Circuits 51 ...
Page 52: ...Fig A1 13 CoBra Microcomputer Keyboard Interfacing Circuit 52 ...
Page 53: ...Fig A1 14 CoBra Microcomputer TV Monitor Interfacing Circuit 53 ...
Page 55: ...10 APPENDIX 2 Flopppy Disk Interface Schematics 55 ...
Page 56: ...Fig A2 1 Floppy Disk Interface Disk Controller 56 ...
Page 57: ...Fig A2 2 Floppy Disk Interface Command and Control Signals Generator Circuits 57 ...
Page 58: ...Fig A2 3 Floppy Disk Interface Write Clock and Digital PLL Circuits 58 ...
Page 59: ...10 APPENDIX 3 Component Placement on Boards 59 ...
Page 60: ...60 Fig A3 1 Component Placement on Keyboard Circuit Board ...
Page 61: ...61 Fig A3 3 Keyboard Circuit Board top layer seen from above keys side ...
Page 62: ...62 Fig A3 3 Keyboard Circuit Board bottom layer seen from above keys side ...
Page 63: ...Fig A3 5 Component placement on the floppy interface board 63 ...
