Commodore 128D User Manual download pdf (Page 24)

Address Description

$D500 CR Conﬁguration Register

$D501 PCRA Preconﬁguration Register A

$D502 PCRB Preconﬁguration Register B

$D503 PCRC Preconﬁguration Register C

$D504 PCRD Preconﬁguration Register D

$D505 MCR Mode Conﬁguration Register

$D506 RCR RAM Conﬁguration Register

$D507 P0L Page 0 pointer low

$D508 P0H Page 0 pointer high

$D509 P1L Page 1 pointer low

$D50A P1H Page 1 pointer high

$D50B SVR System Version Register

$FF00 CR Conﬁguration Register

$FF01 LCRA Load Conﬁguration Register A

$FF02 LCRB Load Conﬁguration Register B

$FF03 LCRC Load Conﬁguration Register C

$FF04 LCRD Load Conﬁguration Register D

Table 7: The MOS 8722 MMU registers

techniques if you want to determine the real amount of memory banks on the system. Actually, the only difference

between MMU expanded systems and unexpanded ones is that the banks 2 and 3 map to banks 0 and 1 on stock

C128’s.

The Conﬁguration Register (Table 8) reﬂects the memory conﬁguration state upon reading, and whenever you

write to it, the new memory conﬁguration will be selected immediately. If you have selected the I/O block at $D000,

you can access the CR at $D500, but the address $FF00 is available in all memory conﬁgurations. Generally, the

processor addresses $FF00–$FF04 are always mapped to the MMU.

There are also Preconﬁguration registers, which let you to change conﬁgurations easier. If you write anything

to a Load Conﬁguration Register, the value in the corresponding Preconﬁguration Register will be loaded to the

Conﬁguration Register, thus selecting that memory conﬁguration. Upon reading, the Load Conﬁguration Registers

reﬂect the state of the corresponding Preconﬁguration Register. These registers are normally used by the BASIC

interpreter, so if you want your routines to coexist with the BASIC, it is probably best to leave those registers alone.

The Mode Conﬁguration Register (Table 9) lets you to change processors and operating modes, among others. All

bits in this register act as outputs, but they can be used as inputs, too. If you wrote a ‘1’ to any bit, external hardware

could pull the line down, and the bit would output ‘0’. For instance, you can make the operating system to think

that the 40/80 key is depressed by clearing the high bit of this register. Also, if you make your own routine to select

the C64 mode, you will able to assert the EXROM or GAME, if you want to play with different C64 mode memory

conﬁgurations without having to hook anything to the computer. Also, note that you can use other banks than the

default 0 in the C64 mode, if you disable common memory and remember to select the video bank.

The 64 kB video bank can be selected with the RAM Conﬁguration Register (Table 10). Its other purpose is

selecting the size and location of the Common RAM block, a memory area that defaults to bank 0 in spite of the

processor RAM bank settings in the Conﬁguration Registers.

Finally, the page relocation registers let you to relocate the processor pages zero and one (addresses $00–$FF and

$100–$1FF) anywhere in the memory. Well, actually the 8502’s addresses 0 and 1 always map to its built-in I/O

bottom of memory. To relocate the zero page, ﬁrst write the memory bank (0–3) to $D508, and then write the memory

page number to ($D507). The stack page (page 1) uses the addresses $D509 and $D50A.

3.7 Hints for programming in C128 mode Machine Language

It is best to use sixteen or thirty-two kilobytes of common memory on the MMU when using the PIA expansion. This

way you can hold your program in bank 0 in the lowmost or highmost sixteen kilobytes, or both, and you can easily

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Commodore 128D User Manual Page 24

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