Controller Pcb - NEC Advanced Personal Computer System Reference Manual

Controller PCB
Therefore, associated with each display position are a character code (or special-
character code) and an attribute code. Both codes are stored in different physical
areas of the display buffer memory, which is separate from main memory and
accessible to only the GDC. This buffer has four 2K x 8-bit CMOS static RAMs for
the character-code memory and two 2K x 8-bit CMOS static RAMs for the
attribute-code memory.
The logical arrangement of this buffer is seen as a single 4K x 24-bit storage area,
with associated character codes and attribute codes logically concatenated into
single 24-bit words. Specifically, for any given display unit, the bit address of its
character code is identical to that of its attribute code except for Bit 12. Bit 12 is '0'
for the character-code address and '1' for the attribute-code address. For example,
character-code address '0000' is associated with attribute-code address '1000'
because their addresses are identical except for Bit 12. Figure 3-6 is a memory map
that shows the organization of the display buffer memory. Notice that Bit 12
determines whether an address accesses a character code (Bit 12
=
0) or a character
attribute code (Bit 12
=
1).
ADDRESS BUS
HEX
IS 14 13 12 11 10 9 8
7
6
5
4 3 2 I
0
0000 0 0 0
0 0 0 0 0 0 0 0 0 0 0 0 0
0800 0 0 0 0 I 0 0 0 0 0 0 0
0
0 0 0
16 BITS
ADDRESSES
1000
0 0 0 I 0 0 0 0 0 0 0 0
0 0 0 0
EIGHT SUCH
~5~ARACTER-CODE
}
256 CHARACTER- STORAGE UNITS
1800 0 0 0 I
0 0 0 0 0 0 0 0 0 0 0
2000
0 0
I 0 0 0 0 0 0 0 0 0 0 0 0 0
2800 0 0 I 0 0 0 0 0 0 0 0
0 0 0 0
3000
0 0
I 0 0 0 0 0 0 0 0 0 0 0 0
FOOO
0 0 0 0 0 0 0 0 0 0 0 0
F800
1 0 0 0 0 0 0 0 0 0 0 0
FFFF I I I I I I I I I I I
Figure 3-6 Display Buffer Memory Map
8 BITS
16 BITS
---
£.
8 BITS
A TTRIBl'TE-CODE
ADDRESSES
256
CHARACTER-CODE
ADDRESSES
256 CHARACTER-
ATTRlBl:TE-CODE
ADDRESSES
3-7