Sharp MZ-800 Service Manual page 16

MZ-800
Concept of the scroll control circuit
Scroll method
• Scrolling by means of VRAM address conversion.
Range of scroll
• y-axis programmable.
BASIC console command compatible
• x-axis fixed
Scroll sequence
• The scroll start address is termed "SSA" and end
address "SEA".
• Execution of scroll, with offset given from the CPU.
• One line (line S) starting from SSA disappears from
the display screen.
• A new line (line S') is added to SEA. Line S' is the
same refresh memory as the line S. The contents of
the memory was erased (nullified by the CPU) before
the execution.
x
Fig-a Scroll area _ _ _ _ _ _ _ _ _ (640/320)
SS'/! ABCDE ABC
1 23456
1234
r--
XYZ
Line
S
~
OPORSTU
9876543
Fig-b Screen before scroll
SSA
ABCDE
A BC
-
XYZ
OPORSTU
SEA
I--
9876543
Line
S'
Fig-c Line after scroll
15
Execution of scrolling
by
address conversion
• Scroll offset (SOF) is the count of lines which the CPU
gives to the CRTC. For instance, the following must
be observed to perform scrolling.
3-line scroll: SOF
3
=
OF x 3
5-line scroll: SOF 5
=
OF x 5
And, to scroll one more line after 5-line scroll;
5-line scroll: SOF5'
=
SOF5
+
OF
=
OF x 6
Display screen
000
SOF
SSA
t - - - - ' ' - - - - - - - - - - - - - i
A Scroll screen
o
SW
SEA
1-----1
OB
IF400 (FAOO)
• Display address DA is the signal created in the CRTC
display address generation circuit and arranged in
their order from the upper left corner of the screen.
The bottom right address is 1 F400 in the 640 x 200
mode.
• Display memory address DMA represents the VRAM
address corresponding to DA.
Since scroll is executed by means of address conver-
sion, the order of DMA may not be the same as DA,
necessarily.
• CPU address MA is the VRAM address that obtained
from the CPU through the CRTC. To lighten burden
on the CPU, a circuit is added to make order of DA
identical to order of MA arrangement.
VRAM
~DMA
Fig-d Address conversion
4-2-6. VRAM data input/output circuit
1. Nothing intervenes for input and output of data in the
case of the MZ-700 mode.
2. MZ-800 mode
• Write
Read data (RD) from the VRAM and write data (WD)
from the CPU are subjected to logical operation
accordi'1~
to the direction from the write format
register (WF) and its result is written.