RAM

8 RAM chips on a C64 board

RAM is a shortcut for Random Access Memory. The chips of computers, which are used as temporary memory, are called RAM. The input - also programs and data - will be loaded into the ram before the CPU can run this program. RAM-Chips can also be used as caches in harddisks, CD- or DVD-drives or as video ram (VRAM).

The C64 has a RAM of 64 KBytes (65536 Bytes). Useable are approx. 39 KB for BASIC-programs. In the original version 8 RAM-chips are used with 8 KByte. The chip-synonym is 4164. In the newer version C64C there are only 2 RAM-chips used with 32 KByte (chip-synonym 4464).

DRAM

The RAM of the C64 is so-called DRAM (shortcut for Dynamic RAM). The information of a DRAM must be refreshed after a few milliseconds.

SRAM

The chip-synonym of the colour RAM is 2114 with 0,5 KByte. The colour RAM used SRAM (shortcut for Static RAM). This RAM-type does not need a refresh.

REU

The RAM of the C64 can be enhanced with cartridges. The famous Memory Cartridge was built by Commodore and is called REU (RAM Expansion Unit). Another Memory cartridge is called GeoRAM, which is only used with the operating system GEOS.

Pinouts of RAM chips in the C-64

2114 1K x 4 SRAM.
One of these makes up the text screen color RAM at 55296–56295/$D800–DBE7. Three of these chips (expandable by another 8) make up the main memory in the VIC-20.

     +---+--+---+
  A6 |1  +--+ 18| VCC
  A5 |2       17| A7
  A4 |3       16| A8
  A3 |4       15| A9
  A0 |5  2114 14| D1
  A1 |6       13| D2
  A2 |7       12| D3
 !CS |8       11| D4
 GND |9       10| !WE
     +----------+

4116 16 K x 1 DRAM.
Needs −5 (VEE), +5 and +12 (VDD) volt power supply.

     +---+--+---+
 VEE |1  +--+ 16| GND
   D |2       15| !CAS
 !WE |3       14| Q
!RAS |4  4116 13| A6
  A0 |5       12| A3
  A2 |6       11| A4
  A1 |7       10| A5
 VDD |8        9| VCC
     +----------+

How to use a 4164 chip instead the original 4116:

• Bend pin 1 and 8 upwards so they no longer go into the socket's connectors.
• Connect Pin 8 and 9 with one another through a short piece of wire.
• Make sure pin 1 and 8 are not in contact with any other components nearby.
• Insert the chip like this into the socket.

4164 or 4264 64 K x 1 DRAM.

     +---+--+---+
 N/C |1  +--+ 16| GND
   D |2       15| !CAS
 !WE |3       14| Q
!RAS |4  4164 13| A6
  A0 |5  4264 12| A3
  A2 |6       11| A4
  A1 |7       10| A5
 VCC |8        9| A7
     +----------+

If you need one of the 4164 chips, you can also use a 41256 instead. You have to solder a short piece of wire between pins 1 and 16 of that chip (these are the pins just left and right of the alignment notch on the chip). This mod will make the chip look just like a '64 chip to the system.

4464 64 K x 4 DRAM.

     +---+--+---+
 !OE |1  +--+ 18| GND
  D0 |2       17| D3
  D1 |3       16| !CAS
 !WE |4       15| D2
!RAS |5  4464 14| A0
  A6 |6       13| A1
  A5 |7       12| A2
  A4 |8       11| A3
 VCC |9       10| A7
     +----------+

41256, 41257 256 K x 1 DRAM.

     +---+--+---+
  A8 |1  +--+ 16| GND
   D |2       15| !CAS
 !WE |3       14| Q
!RAS |4 41256 13| A6
  A0 |5 41257 12| A3
  A2 |6       11| A4
  A1 |7       10| A5
 VCC |8        9| A7
     +----------+

44256, 44258 256 K x 4 DRAM.

     +---+--+---+
  D0 |1  +--+ 20| GND
  D1 |2       19| D3
 !WE |3       18| D2
!RAS |4       17| !CAS
 N/C |5 44256 16| !OE
  A0 |6 44258 15| A8
  A1 |7       14| A7
  A2 |8       13| A6
  A3 |9       12| A5
 VCC |10      11| A4
     +----------+

Cross reference table

With the help of the cross reference table one can see the classifications of each of the producers for RAM chips. Although they are electrically and mechanically identical, every producer has his own system to mark the RAM chips. While the preceding letters often hint to the producer, the following series of digits indicates the size and organisation of the memory. Next, there are often one or more letters, which most stand for the housing shape. Finally, there is again one or more digits, e.g. "-15", this tells you something about the maximum access speed.

Example: 2114L2 - this memory chip has an access time of 120 ns (nano seconds) and can without any problems be exchanged by a faster chip 2114L1 (100ns), but not by a 2114L3 (150ns) or 2114L4 (200ns). As also today with the PC, you have to not only choose the appropriate memory size, but you also have to know where the limit for the access time is.

For the 4164 DRAM, many C64 has DRAM´s marked with 150nS and 200nS speed.

Memory chips that are built into Commodore computers
Type Producer	2114 1024 x 4 SRAM	4164 64K x 1 DRAM
Hitachi	HM 472114	HM 4864
Intel	P2114
OKI	MSM2114-20	M 3764 A
Micron		MT4264
Mitsubishi	MSM2114 RS-3	M5K4164 ANP-10
Motorola	MCM 21L14-20	MCM 6665 AP
NEC	uPD2114LC5	uPD 4164-1
Solid State Scientific	SCM2114AL
Toshiba	TC 5114 AD-2 & AP-2	TMM 4164 C-3

U6 Color SRAM Failure Symptoms

• Startup screen shows flickering characters with shimmering colors.
• Also check the [PLA] chip at U17.

Links

Wikipedia: Random_Access_Memory

Hardware internals of the C64 · Hardware portal
Components of the C64: CPU · VIC · SID · CIA · PLA · RAM · ROM · Color RAM

Interfaces of the C64: Control Port · Expansion Port · Userport · Cassette Port · Power Supply Connector · A/V Jack · RF Jack · Serial Port