A SIMM, or single in-line memory module, is a type of memory module containing random access memory used in computers from the early 1980s to the late 1990s. It differs from a dual in-line memory module (DIMM), the most predominant form of memory module today, in that the contacts on a SIMM are redundant on both sides of the module. SIMMs were standardised under the JEDEC JESD-21C standard.

Most early PC motherboards (8088-based PCs and XTs) used socketed DIP chips. With the introduction of 286-based PC/ATs, which could use larger amounts of memory, memory modules evolved to save motherboard space and to ease memory expansion. Instead of plugging in eight or nine single DIP DRAM chips, only one additional memory module was needed to increase the memory of the computer. A few 286-based computers used (often non-standard) memory modules like SIPP memory (single in-line pin package). The SIPP's 30 pins often bent or broke during installation, which is why they were quickly replaced by SIMMs which used contact plates rather than pins.

SIMMs were invented and patented by Wang Laboratories. One of Wang's scientists, James Clayton, invented what was to become the basic memory module, now known as a SIMM (single in-line memory module) in 1983. The first SIMMs appeared on the PS/2 in the mid 1980s, having been first proposed by Skip Coppola while at IBM. They solved several problems at the time, including shrinking motherboard real estate (they took up much less board space than socketed chips) as well as the effects of rapidly advancing memory capacities (a motherboard would quickly become obsolete based on its sockets for a particular RAM chip capacity) This also allowed the manufacturer (IBM at this time) to source RAM chips from different vendors and in different packaging, yet still allow them to be interchangeable through this intermediate form (the SIMM).

The first variant of SIMMs has 30 pins and provides 8 bits of data (9 bits in parity versions). They were used in 286, 386 and 486 systems.

The second variant of SIMMs has 72 pins and provides 32 bits of data (36 bits in parity versions). These appeared on 486, Pentium, Pentium Pro and even some Pentium II systems. By the mid 90s, 72-pin SIMMs had replaced 30-pin SIMMs.

Non-IBM PC computers such as UNIX workstations may use proprietary non-standard SIMMs. The Macintosh IIfx use proprietary non-standard SIMMs with 64 pins.

DRAM technologies used in SIMMs include EDO and FPM.

Due to the differing data bus widths of the memory modules and some processors, sometimes several modules must be installed in identical pairs or in identical groups of four to fill a memory bank. The general rule of thumb is a 286 or 386SX system (data bus width of 16 bits) would require two 30-pin SIMMs for a memory bank. On 386DX or 486 systems (data bus width of 32 bits), either four 30-pin SIMMs or one 72-pin SIMM are required for one memory bank. On Pentium systems (data bus width of 64 bits), two 72-pin SIMMs are required. However, some Pentium systems have support for a "half bank mode", in which the data bus would be shortened to only 32-bits to allow operation of a single SIMM. Conversely, some 386 and 486 systems use what is known as "memory interleaving", which requires twice as many SIMMs and effectively doubles the bandwidth.

The earliest SIMM sockets were conventional push-type sockets. These were soon replaced by ZIF sockets in which the SIMM was inserted and rotated until it locked into place. To install a SIMM, the module must be placed in the socket at an angle, then rotated (angled) into position. To remove one, the two metal or plastic clips at each end must be pulled to the side, then the SIMM must be tilted back and pulled out. The earlier sockets used plastic retainer clips which were found to break, so steel clips replaced them.

Standard sizes available

30-pin SIMM: 256 KB, 1 MB, 4 MB, 16 MB

72-pin SIMM: 1 MB, 2 MB, 4 MB, 8 MB, 16 MB, 32 MB, 64 MB, 128 MB, 256 MB

Standard pinouts

30-pin SIMM Memory Module

Pin #	Nomenclature	Signal Description
1	VCC	+5VDC
2	CAS	Column Address Strobe
3	DQ1	Data 0
4	A0	Address 0
5	A1	Address 1
6	DQ2	Data 1
7	A2	Address 2
8	A3	Address 3
9	VSS	Ground
10	DQ3	Data 2
11	A4	Address 4
12	A5	Address 5
13	DQ4	Data 3
14	A6	Address 6
15	A7	Address 7
16	DQ5	Data 4
17	A8	Address 8
18	A9	Address 9
19	A10	Address 10
20	DQ6	Data 5
21	W	Write Enable
22	VSS	Ground
23	DQ7	Data 6
24	NC	No Internal Connection
25	DQ8	Data 7
26	NC	No Internal Connection
27	RAS	Row Address Strobe
28	NC	No Internal Connection
29	NC	No Internal Connection
30	VCC	+5VDC

72-pin ECC SIMM Memory Module

Pin #	Non-Parity	Parity	Signal Description
1	VSS	VSS	Ground
2	DQ0	DQ0	Data 0
3	DQ1	DQ1	Data 1
4	DQ2	DQ2	Data 2
5	DQ3	DQ3	Data 3
6	DQ4	DQ4	Data 4
7	DQ5	DQ5	Data 5
8	DQ6	DQ6	Data 6
9	DQ7	DQ7	Data 7
10	VCC	VCC	+5 VDC
11	PD1	PD1	Presence Detect 1
12	A0	A0	Address 0
13	A1	A1	Address 1
14	A2	A2	Address 2
15	A3	A3	Address 3
16	A4	A4	Address 4
17	A5	A5	Address 5
18	A6	A6	Address 6
19	A10	A10	Address 10
20	n/c	PQ8	Data 8 (Parity 1)
21	DQ9	DQ9	Data 9
22	DQ10	DQ10	Data 10
23	DQ11	DQ11	Data 11
24	DQ12	DQ12	Data 12
25	DQ13	DQ13	Data 13
26	DQ14	DQ14	Data 14
27	DQ15	DQ15	Data 15
28	A7	A7	Address 7
29	A11	A11	Address 11
30	VCC	VCC	+5 VDC
31	A8	A8	Address 8
32	A9	A9	Address 9
33	/RAS3	RAS3	Row Address Strobe 3
34	/RAS2	RAS2	Row Address Strobe 2
35	DQ16	DQ16	Data 16
36	n/c	PQ17	Data 17 (Parity 2)
37	DQ18	DQ18	Data 18
38	DQ19	DQ19	Data 19
39	VSS	VSS	Ground
40	/CAS0	CAS0	Column Address Strobe 0
41	/CAS2	CAS2	Column Address Strobe 2
42	/CAS3	CAS3	Column Address Strobe 3
43	/CAS1	CAS1	Column Address Strobe 1
44	/RAS0	RAS0	Row Address Strobe 0
45	/RAS1	RAS1	Row Address Strobe 1
46	A12	A12	Address 12
47	/WE	WE	Read/Write
48	A13	A13	Address 13
49	DQ20	DQ20	Data 20
50	DQ21	DQ21	Data 21
51	DQ22	DQ22	Data 22
52	DQ23	DQ23	Data 23
53	DQ24	DQ24	Data 24
54	DQ25	DQ25	Data 25
55	n/c	PQ26	Data 26 (Parity 3)
56	DQ27	DQ27	Data 27
57	DQ28	DQ28	Data 28
58	DQ29	DQ29	Data 29
59	DQ31	DQ31	Data 31
60	DQ30	DQ30	Data 30
61	VCC	VCC	+5 VDC
62	DQ32	DQ32	Data 32
63	DQ33	DQ33	Data 33
64	DQ34	DQ34	Data 34
65	n/c	PQ35	Data 35 (Parity 4)
66	PD2	PD2	Presence Detect 2
67	PD3	PD3	Presence Detect 3
68	PD4	PD4	Presence Detect 4
69	PD5	PD5	Presence Detect 5
70	PD6	PD6	Presence Detect 6
71	PD7	PD7	Presence Detect 7
72	VSS	VSS	Ground

See also

• Dual in-line package (DIP)
• Single in-line package (SIP)
• Zig-zag in-line package (ZIP)
• Dual in-line memory module (DIMM)

External links

• General SIMM Installation Guide