Commodore 128D User Manual download pdf (Page 6)

64 kilobit DRAMs required 128 refresh cycles every 2 milliseconds, now the 256 kb chips need 256 cycles but every

4 ms.

Whenever you select a row address,

the block to which the row address belongs gets refreshed. As the 64 kilobit

chips have a 7-bit refresh counter, the lowest seven row address bits specify the row address, and the highest bit is

ignored. The 256 kilobit memory chips have an 8-bit counter, so they ignore the 9th row address bit and determine the

block to be refreshed by the eight lowest bits.

The VIC-IIe chip refreshes the memory systematically, 5 rows in the end of each screen scan line. It does this

by selecting a row address determined by its internal counter, and then increases this counter by one. The video chip

could have only 7-bit refresh counter, and it would still operate with 64 kb chips, but fortunately it has an 8-bit counter,

so all of the 256 kb chips get refreshed.

Newer memory chips can be refreshed using a CAS-before-RAS technique. In this technique, you pull ﬁrst the

CAS signal low, and then the RAS signal. The memory chips recognize this as a memory refresh condition, and

they refresh a block and increase their internal refresh counter. However, this technique was not available when the

Commodore 64 and its video chip were designed.

1.5 The MMU expansion

The Commodore 128 has two memory banks, numbered 0 and 1. The banks are switched in and out by a custom chip

called MOS 8722 MMU (Memory Management Unit). The chip has the registers for handling four memory banks, but

there are only two hardware lines for bank selection, named CAS0 and CAS1. They are connected to the CAS signal

of the memory chips in banks 0 and 1, respectively.

The MMU expansion adds two new memory banks to the computer. It adds another 8722 MMU chip to the system,

routing some signals so that the chip considers bank 2 as bank 1. The CAS0 and CAS1 outputs of the two MMU chips

will be combined to form the CAS signals for all four memory banks. The logic glue involved is very simple, and

designing it was quite straight-forward.

2 Building the expansion

2.1 Disclaimer

Although this procedure worked perfectly for me, I cannot guarantee that anyone else can perform this upgrade without

damaging their computer. I therefore disclaim any responsibilty for any damage that may occur as a result of attempting

this upgrade. It will also void any warranty on your computer.

On a more positive note, there is no reason why someone who is experienced in wielding a soldering iron, and has

done some electronic construction or troubleshooting, should not be able to perform this upgrade successfully.

2.2 Getting started

A termostate soldering iron, desoldering pump or other desoldering tool, a screwdriver, a spoon and a continuity

tester are the only tools needed. The spoon is for removing the chips. A bottle top remover is not suitable for that.

The continuity tester is vital for checking suspicious connections. If your tester does not automatically select proper

measuring range, use the coarsest (MΩ) range, as it uses smallest current, which shouldn’t damage any chips on the

board.

The installation begins of course by opening the machine and removing the keyboard and LED cables (and internal

drive and power supply in the C128D). It is useful to memorize, photograph or draw how the parts were initially

connected.

After removing the cables, open the screws that hold the metal RF shield and the motherboard with the case, and

remove the shield and the board.

See Section 1.3.

A tiny screwdriver is equally good. Just insert the screwdriver tip under one end of the chip and wound it a bit in upward angle so that the chip

moves slightly. Then insert it to the other end of the chip and try to lift it a bit. You may have to repeat this procedure. Be careful not to wound the

pins too much.

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Commodore 128D User Manual Page 6

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