Commodore 128 Programmer's Reference Manual page 223

THE POWER BEHIND COMMODORE 128 GRAPHICS 213
I I
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In essence, the bank concept can be thought of in this way: The C128 has a 16K
(VIC) video bank within a selected memory configuration within a 64K RAM bank.
Figures 8-28 through 8-32 at the end of this chapter provides a graphics program
ming summary.
SHADOW REGISTERS: INTERMEDIATE STORAGE
LOCATIONS USED BY THE CI28 SCREEN EDITOR
Users who are experienced in programming the Commodore 64 VIC chip will find that
most of the graphics operations of the Commodore 128 are performed in the same way
as the C64. The main difference between the Commodore 64 and the Commodore 128
graphics systems is the hardware implementation of split-screen modes.
C128 mode provides two types of split-screen displays:
1. Standard character mode and bit map mode
2. Standard character mode and multi-color bit map mode
Because the split-screens switch from one display mode to another at a given
time, the screen editor must be interrupt-driven. The interrupt indicates at what point
the mode is to be switched. At that point, the VIC chip is loaded with preset values
already contained in RAM. These preset values are known as shadow registers. Each
time an interrupt occurs, certain video-chip registers are cleared and refreshed with the
values in the shadow registers. These shadow registers add a variation in programming
the VIC chip compared to the way the Commodore 64 handles it.
The primary intermediate storage locations for VIC chip programming are:
n
ACTUAL LOCATION
BIT 4 - 53270 ($D016)
NAME INDIRECT LOCATION
GRAPHM BIT 7 - 216 ($00D8)
GRAPHM Bit 6 - 216 ($00D8)
GRAPHM BIT 5 - 216 ($00D8) BIT 5 - 53265 ($D011)
VM1* BITS 7-4 - 2604 ($0A2C) BITS 7-4 - 53272 ($D018)
VM1 BITS 3-0 - 2604 ($0A2C) BITS 3-0 - 53272 ($D018)
VM2** BITS 7-4 - 2605 ($0A2D) BITS 7-4 - 53272 ($D018)
VM2 BITS 3-0 - 2605 ($0A2D) BITS 3-0 - 53272 ($D018)
*VMI applies only to standard and multi-color character (text) modes.
**VM2 applies only to standard and multi-color bit map modes.
DESCRIPTION
Multicolor Mode Bit
Split Screen Bit
Bit Map Mode Bit
Video Matrix (screen memory) Pointer
Character Base Pointer
Video Matrix (screen memory) Pointer
Bit Map Pointer