Commodore 128D User Manual download pdf (Page 26)

RAM Conﬁguration Register

Bits Function

7–6 Video RAM bank (0–3)

5–4 RAM block (0–3)

3–2 Common RAM selection

00 Common RAM block disabled

01 Common RAM block at bottom of memory

10 Common RAM block at top of memory

11 Common RAM block at both top and bottom

1–0 Size of Common RAM block

00 1 kB

01 4 kB

10 8 kB

11 16 kB

Table 10: The MOS 8722 MMU RAM Conﬁguration Register ($D506)

System Version Register

Bits Description

7–4 Bank version (amount of 64 kB banks)

3–0 MMU chip version

Table 11: The MOS 8722 MMU System Version Register ($D50B)

access all of the memory through two 16-kilobyte windows at $4000 and $8000. However, keep in mind that the page

relocation cannot be used outside bank 0 when the low common memory block has been selected.

If you are working with graphics, switch the character generator ROM off, and you have one restriction less. The

three lowmost bits of the processor’s built-in I/O register (at addresses 0 and 1) have totally different function in the

C128 mode. The lowmost bit, LORAM, selects one of the two color memory banks for the processor. The second bit

from right, HIRAM, selects the color memory bank for the video chip. The CHAREN bit, bit 2, dictates whether the

character generator ROM is mapped to the video chip addresses $1000–$1FFF or not.

3.8 Programming in C128 mode BASIC

Due to the complexity of the C128’s operating system, it is very difﬁcult, if not impossible, to utilize the PIA expansion

memory in BASIC 7.0. You have to leave the segments 0 and 3 alone, as they contain system variables, routines to

access different RAM banks, and interrupt vectors. The other segments are not safe either, as the BASIC interpreter

uses all of the RAM bank 1 for BASIC variables. If you switch other RAM blocks to a segment, you will have to

ensure that no BASIC program data or variables occupy any of the address range used by that segment.

For this reason, I did not even try to ﬁnd out how you could utilize the PIA expansion in the BASIC 7.0. But

you can use the MMU expansion RAM very easily with the built-in BANK command. The new RAM banks have the

numbers 2 and 3.

If you really want to utilize the PIA expanded memory in BASIC 7.0 programs, it is possible by using machine

language subroutines to access the extra memory. Remember to disable the interrupts in your subroutine, as the BASIC

interpreter uses raster interrupts.

4 Programming the expansion in C64 mode BASIC

With BASIC 2.0 the use of the extra memory is a bit limited. In the upmost segment (segment 3) there is operating

system ROM, under which you can place different memory blocks, but reading them with BASIC is naturally impossi-

ble. However, in some cases writing data to this segment partially under Kernal ROM and I/O area may be a working

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Commodore 128D User Manual Page 26

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