1. Dram Memory Slot (x2) Function Tool
2. Dram Memory Slot (x2) Function Chart
3. Dram Memory Slot (x2) Functions
4. Dram Memory Slot (x2) Functioning

DRAM Memory Tutorial Includes:
DRAM memory technologyHow does DRAM workDRAM types
Return to:Memory types & technologies

DRAM is a form of semiconductor memory, but it operates in a slightly different way to other formats.

In order to be able to design and use DRAM, it is obviously wise to be able to have an understanding about the DRAM operation and its functionality.

The concept of memory rank applies to all memory module form factors, though in general it tends to matter primarily on server platforms, due to the larger amounts of memory they manage. A memory rank is a block or area of data that is created using some, or all, of the memory chips on a module. A rank is a data block that is 64 bits wide. Crucial Ballistix 3200 MHz DDR4 DRAM Desktop Gaming Memory Kit 16GB (8GBx2) CL16 BL2K8G32C16U4B (Black) 4.8 out of 5 stars 5,693. Yes, DRAM is a short term memory. It is Dynamic RAM, which means when the power goes off, it looses its contents. SRAM would be Static RAM, which means it saves its memory contents after power.

DRAM operation basics

DRAM memory technology has MOS technology at the heart of the design, fabrication and operation. Looking at how a DRAM memory works, it can be see that the basic dynamic RAM or DRAM memory cell uses a capacitor to store each bit of data and a transfer device - a MOSFET - that acts as a switch.

The level of charge on the memory cell capacitor determines whether that particular bit is a logical '1' or '0' - the presence of charge in the capacitor indicates a logic '1' and the absence of charge indicates a logical '0'.

The basic dynamic RAM memory cell has the format that is shown below. It is very simple and as a result it can be densely packed on a silicon chip and this makes it very cheap.

Two lines are connected to each dynamic RAM cell - the Word Line (W/L) and the Bit Line (B/L) connect as shown so that the required cell within a matrix can have data read or written to it.

The basic memory cell shown would be one of many thousands or millions of such cells in a complete memory chip. Memories may have capacities of 256 Mbit and more. To improve the write or read capabilities and speed, the overall dynamic RAM memory may be split into sub-arrays. The presence of multiple sub-arrays shortens the word and bit lines and this reduces the time to access the individual cells. For example a 256 Mbit dynamic RAM, DRAM may be split into 16 smaller 16Mbit arrays.

The word lines control the gates of the transfer lines, while the bit bines are connected to the FET channel and are ultimately connected to the sense amplifiers.

There are two ways in which the bit lines can be organised:

• Folded Bit Lines: It is possible to consider a pair of adjacent bit lines as a single bit line folded in half with the connection on the fold broken and connected to a shared sense amplifier. This format provides additional noise immunity, but at the expense of being less compact.
• Open Bit Lines: In this configuration the sense lines are placed between two sub-arrays, thereby connecting each sense amplifier to one bit line in each array. This offers a more compact solution than the folded bit lines, but at the expense of noise immunity.

Dynamic RAM read / write operation

One of the critical issues within the dynamic RAM is to ensure that the read and write functions are carried out effectively. As voltages on the charge capacitors are small, noise immunity is a key issue.

There are several lines that are used in the read and write operations:

• /CAS, the Column Address Strobe: This line selects the column to be addressed. The address inputs are captured on the falling edge of /CAS. It enables a column to be selected from the open row for read or write operations.
• /OE, Output Enable: The /OE signal is typically used when controlling multiple memory chips in parallel. It controls the output to the data I/O pins. The data pins are driven by the DRAM chip if /RAS and /CAS are low, /WE is high, and /OE is low. In many applications, /OE can be permanently connected low, i.e. output always enabled if not required for example of chips are not wired in parallel.
• /RAS, the Row Address Strobe: As the name implies, the /RAS line strobes the row to be addressed. The address inputs are captured on the falling edge of the /RAS line. The row is held open as long as /RAS remains low.
• /WE, Write Enable: This signal determines whether a given falling edge of /CAS is a read or write. Low enables the write action, while high enables a read action. If low (write), the data inputs are also captured on the falling edge of /CAS.

Dynamic RAM refresh operation

One of the problems with this arrangement is that the capacitors do not hold their charge indefinitely as there is some leakage across the capacitor. It would not be acceptable for the memory to lose its data, and to overcome this problem the data is refreshed periodically. The data is sensed and written and this then ensures that any leakage is overcome, and the data is re-instated.

One of the key elements of DRAM memory is the fact that the data is refreshed periodically to overcome the fact that charge on the storage capacitor leaks away and the data would disappear after a short while. Typically manufacturers specify that each row should be refreshed every 64 ms. This time interval falls in line with the JEDEC standards for dynamic RAM refresh periods.

There are a number of ways in which the refresh activity can be accomplished. Some processor systems refresh every row together once every 64 ms. Other systems refresh one row at a time, but this has the disadvantage that for large memories the refresh rate becomes very fast. Some other systems (especially real time systems where speed is of the essence) adopt an approach whereby a portion of the semiconductor memory at a time based on an external timer that governs the operation of the rest of the system. In this way it does not interfere with the operation of the system.

Whatever method is use, there is a necessity for a counter to be able to track the next row in the DRAM memory is to be refreshed. Some DRAM chips include a counter, otherwise it is necessary to include an additional counter for this purpose.

It may appear that the refresh circuitry required for DRAM memory would over complicate the overall memory circuit making it more expensive. However it is found that DRAM the additional circuitry is not a major concern if it can be integrated into the memory chip itself. It is also found that DRAM memory is much cheaper and has a much greater capacity than the other major contender which might be Static RAM (SRAM).

DRAM size

As the size of memories increases, the issue of signal to noise ratio becomes very important. At first sight, this may not appear to be a major issue, but it can give rise to issues of data corruption.

The signal to noise ratio depends upon the ratio of the capacitance of the storage capacitor within the DRAM memory to the capacitance of the Word or Bit line on which the charge is dumped when the cell is accessed. As the bit density per chip is increased, the ratio is degraded since the cell area is decreased as more cells are added on the bit line. It is for this reason that it is important to store as high a voltage on the cell capacitor, and also to increase the capacitance of the DRAM storage capacitor for a given areas as much as possible.

This is a very important consideration because sensing the small charge on the memory cell capacitor is one of the most challenging areas of the DRAM memory chip design. As a result of this some elaborate circuit designs have been incorporated onto DRAM memory chips.

DRAM memory chips are widely used and the technology is very well established. It has become very reliable and DRAM memory chips and plug in boards are available to expand the memory of computers and many other devices. Although DRAM has its disadvantages, it is still widely used because it offers many advantages in terms of cost size and a satisfactory speed - it is not he fastest, but still faster than some types of memory.

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Choose your iMac model

If you're not sure which iMac you have, you can identify your iMac and then select it from the list below.

27-inch

24-inch

Dram Memory Slot (x2) Function Tool

21.5-inch

• iMac (Retina 4K, 21.5-inch, 2019)*
• iMac (Retina 4K, 21.5-inch, 2017)*
• iMac (21.5-inch, 2017)*
• iMac (21.5 inch, Mid 2014)*
• iMac (21.5-inch, Late 2013)*
• iMac (21.5-inch, Late 2012)*

20-inch

17-inch

* Memory is not removable by users on iMac (21.5-inch, Late 2012), iMac (21.5-inch, Late 2013), iMac (21.5-inch, Mid 2014), iMac (21.5-inch, 2017), iMac (Retina 4K, 21.5-inch, 2017) and iMac (Retina 4K, 21.5-inch, 2019). If the memory in one of these computers needs repair service, contact an Apple Retail Store or Apple Authorised Service Provider. If you’d like to upgrade the memory in one of these models, an Apple Authorised Service Provider can help. Before you schedule an appointment, confirm that the specific Apple Authorised Service Provider offers memory upgrade services.

Memory in iMac (21.5-inch, Late 2015) and iMac (Retina 4K, 21.5-inch, Late 2015) is not upgradeable.

iMac (Retina 5K, 27-inch, 2020)

Get memory specifications for iMac (Retina 5K, 27-inch, 2020), then learn how to install memory in this model.

Memory specifications

This iMac model features Synchronous Dynamic Random-Access Memory (SDRAM) slots on the rear of the computer near the vents with these memory specifications:

For optimal memory performance, DIMMs should be the same capacity, speed and supplier. Use Small Outline Dual Inline Memory Modules (SO-DIMM) that meet all of these criteria:

• PC4-21333
• Unbuffered
• Nonparity
• 260-pin
• 2666MHz DDR4 SDRAM

If you have mixed capacity DIMMs, see the install memory section for installation recommendations.

iMac (Retina 5K, 27-inch, 2019)

Get memory specifications for iMac (Retina 5K, 27-inch, 2019), then learn how to install memory in this model.

Memory specifications

This iMac model features Synchronous Dynamic Random-Access Memory (SDRAM) slots on the rear of the computer near the vents with these memory specifications:

Use Small Outline Dual Inline Memory Modules (SO-DIMM) that meet all of these criteria:

• PC4-21333
• Unbuffered
• Nonparity
• 260-pin
• 2666MHz DDR4 SDRAM

iMac (Retina 5K, 27-inch, 2017)

Get memory specifications for iMac (Retina 5K, 27-inch, 2017), then learn how to install memory in this model.

Memory specifications

This iMac model features Synchronous Dynamic Random-Access Memory (SDRAM) slots on the rear of the computer near the vents with these memory specifications:

Use Small Outline Dual Inline Memory Modules (SO-DIMM) that meet all of these criteria:

• PC4-2400 (19200)
• Unbuffered
• Nonparity
• 260-pin
• 2400MHz DDR4 SDRAM

iMac (Retina 5K, 27-inch, Late 2015)

Get memory specifications for iMac (Retina 5K, 27-inch, Late 2015), then learn how to install memory in this model.

Memory specifications

This iMac model features Synchronous Dynamic Random-Access Memory (SDRAM) slots on the rear of the computer near the vents with these memory specifications:

Use Small Outline Dual Inline Memory Modules (SO-DIMM) that meet all of these criteria:

• PC3-14900
• Unbuffered
• Nonparity
• 204-pin
• 1867MHz DDR3 SDRAM

For these 27-inch models

Get memory specifications for the following iMac models, then learn how to install memory in them:

• iMac (Retina 5K, 27-inch, Mid 2015)
• iMac (Retina 5K, 27-inch, Late 2014)
• iMac (27-inch, Late 2013)
• iMac (27-inch, Late 2012)

Memory specifications

These iMac models feature Synchronous Dynamic Random-Access Memory (SDRAM) slots on the rear of the computer near the vents with these memory specifications:

Use Small Outline Dual Inline Memory Modules (SO-DIMM) that meet all of these criteria:

• PC3-12800
• Unbuffered
• Nonparity
• 204-pin
• 1600MHz DDR3 SDRAM

Installing memory

The internal components of your iMac can be warm. If you've been using your iMac, wait ten minutes after shutting it down to let the internal components cool.

After you've shut down your iMac and given it some time to cool down, follow these steps:

1. Disconnect the power cord and all other cables from your computer.
2. Place a soft, clean towel or cloth on the desk or other flat surface to prevent scratching the display.
3. Hold the sides of the computer and slowly lay the computer face-down on the towel or cloth.
4. Open the memory compartment door by pressing the small grey button located just above the AC power port:
5. The memory compartment door will open as the button is pushed in. Remove the compartment door and set it aside:
6. A diagram on the underside of the compartment door shows the memory cage levers and the orientation of the DIMM. Locate the two levers on the right and left sides of the memory cage. Push the two levers outwards to release the memory cage:
7. After the memory cage has been released, pull the memory cage levers towards you, allowing access to each DIMM slot.
8. Remove a DIMM by pulling the module straight up and out. Note the location of the notch on the bottom of the DIMM. When reinstalling DIMMs, the notch must be orientated correctly or the DIMM won't fully insert:
9. Replace or install a DIMM by setting it down into the slot and pressing firmly until you feel the DIMM click into the slot. When you insert a DIMM, make sure you align the notch on the DIMM to the DIMM slot. Find your model below for specific installation instructions and notch locations:
   • iMac (Retina 5K, 27-inch, 2020) DIMMs have a notch on the bottom, slightly to the left of the centre. If your DIMMs are mixed in capacity, minimise the capacity difference between Channel A (slots 1 and 2) and Channel B (slots 3 and 4) when possible.
     
   • iMac (Retina 5K, 27-inch, 2019) DIMMs have a notch on the bottom, slightly to the left of the centre:
   • iMac (27-inch, Late 2012) and iMac (Retina 5K, 27-inch, 2017) DIMMs have a notch on the bottom left:
     
   • iMac (27-inch, Late 2013) and iMac (Retina 5K, 27-inch, Late 2014, Mid 2015, and Late 2015) DIMMs have a notch on the bottom right:
     
10. After you've installed all of your DIMMs, push both memory cage levers back into the housing until they've locked into place:
    
11. Replace the memory compartment door. You don't need to press the compartment door release button when replacing the compartment door.
12. Place the computer in its upright position. Reconnect the power cord and all other cables to the computer, then start up the computer.
    

Your iMac performs a memory initialisation procedure when you first turn it on after upgrading memory or rearranging DIMMs. This process can take 30 seconds or more, and the display of your iMac remains dark until it's finished. Make sure you let the memory initialisation complete.

For these 27-inch and 21.5-inch models

Get memory specifications for the following iMac models, then learn how to install memory in them:

• iMac (27-inch, Mid 2011)
• iMac (21.5-inch, Mid 2011)
• iMac (27-inch, Mid 2010)
• iMac (21.5-inch, Mid 2010)
• iMac (27-inch, Late 2009)
• iMac (21.5-inch, Late 2009)

Memory specifications

Use Small Outline Dual Inline Memory Modules (SO-DIMM) that meet all of these criteria:

i5 and i7 Quad Core iMac computers come with both top memory slots populated. These computers will not start up if only a single DIMM is installed in any bottom slot; these computers should operate normally with a single DIMM installed in any top slot.

Core Duo iMac computers should operate normally with a single DIMM installed in any slot, top or bottom. ('Top' and 'bottom' slots refer to the orientation of the slots in the pictures below. 'Top' refers to the slots closest to the display; 'bottom' refers to the slots closest to the stand.)

Installing memory

The internal components of your iMac can be warm. If you've been using your iMac, wait ten minutes after shutting it down to let the internal components cool.

After you've shut down your iMac and given it some time to cool down, follow these steps:

1. Disconnect the power cord and all other cables from your computer.
2. Place a soft, clean towel or cloth on the desk or other flat surface to prevent scratching the display.
3. Hold the sides of the computer and slowly lay the computer face-down on the towel or cloth.
4. Using a Philips screwdriver, remove the RAM access door in the bottom of your computer:
   
5. Remove the access door and set it aside.
6. Untuck the tab in the memory compartment. If you're replacing a memory module, gently pull the tab to eject any installed memory module:
   
7. Insert your new or replacement SO-DIMM into the empty slot, noting the orientation of the keyway of the SO-DIMM as shown below.
8. After you've inserted it, press the DIMM up into the slot. There should be a slight click when you seat the memory correctly:
   
9. Tuck the tabs above the memory DIMMs, and reinstall the memory access door:
   
10. Place the computer in its upright position. Reconnect the power cord and all other cables to the computer, then start up the computer.

For these 24-inch and 20-inch models

Get memory specifications for the following iMac models, then learn how to install memory in them:

• iMac (24-inch, Early 2009)
• iMac (20-inch, Early 2009)
• iMac (24-inch, Early 2008)
• iMac (20-inch, Early 2008)
• iMac (24-inch Mid 2007)
• iMac (20-inch, Mid 2007)

Memory specifications

These iMac computers have two side-by-side Synchronous Dynamic Random-Access Memory (SDRAM) slots in the bottom of the computer.

Dram Memory Slot (x2) Function Chart

The maximum amount of random-access memory (RAM) you can install in each computer is:

You can use a 1 GB or 2 GB RAM module in each slot for iMac (Mid 2007) and iMac (Early 2008). Use 1 GB, 2 GB or 4 GB modules in each slot for the iMac (Early 2009).

Use Small Outline Dual Inline Memory Modules (SO-DIMM) that meet all of these criteria:

DIMMs with any of the following features aren't supported:

• Registers or buffers
• PLLs
• Error-correcting code (ECC)
• Parity
• Extended data out (EDO) RAM

Installing memory

The internal components of your iMac can be warm. If you've been using your iMac, wait ten minutes after shutting it down to let the internal components cool.

After your iMac has cooled down, follow these steps:

Dram Memory Slot (x2) Functions

1. Disconnect the power cord and all other cables from your computer.
2. Place a soft, clean towel or cloth on the desk or other flat surface to prevent scratching the display.
3. Hold the sides of the computer and slowly lay the computer face-down on the towel or cloth.
4. Using a Philips screwdriver, remove the RAM access door in the bottom of the computer:
   
5. Remove the access door and set it aside.
6. Untuck the tab in the memory compartment. If you're replacing a memory module, untuck the tab and pull it to eject any installed memory module:
   
7. Insert your new or replacement RAM SO-DIMM into the empty slot, noting the orientation of the keyway of the SO-DIMM as shown above.
8. After you've inserted it, press the DIMM up into the slot. There should be a slight click when you seat the memory correctly.
9. Tuck the tabs above the memory DIMMs, and reinstall the memory access door:
   
10. Place the computer in its upright position. Reconnect the power cord and all other cables to the computer, then start up the computer.

For these 20-inch and 17-inch models

Get memory specifications for the following iMac models, then learn how to install memory in them:

• iMac (20-inch Late 2006)
• iMac (17-inch, Late 2006 CD)
• iMac (17-inch, Late 2006)
• iMac (17-inch, Mid 2006)
• iMac (20-inch, Early 2006)
• iMac (17-inch, Early 2006)

Memory specifications

For best performance, fill both memory slots, installing an equal memory module in each slot.
*iMac (Late 2006) uses a maximum of 3GB of RAM.

Installing memory in the bottom slot

The internal components of your iMac can be warm. If you've been using your iMac, wait ten minutes after shutting it down to let the internal components cool.

After you've shut down your iMac and given it some time to cool down, follow these steps:

1. Disconnect the power cord and all other cables from your computer.
2. Place a soft, clean towel or cloth on the desk or other flat surface to prevent scratching the display.
3. Hold the sides of the computer and slowly lay the computer face-down on the towel or cloth.
4. Using a Phillips screwdriver, remove the RAM access door on the bottom of the iMac and set it aside:
   
5. Move the DIMM ejector clips to their fully open position:
   
6. Insert your RAM SO-DIMM into the bottom slot, keeping in mind the orientation of the keyed SO-DIMM:
   
7. After you've inserted it, press the DIMM up into the slot with your thumbs. Don't use the DIMM ejector clips to push in the DIMM, as this may damage the SDRAM DIMM.There should be a slight click when you seat the memory fully.
8. Close the ejector clips:
   
9. Reinstall the memory access door:
   
10. Place the computer in its upright position. Reconnect the power cord and all other cables to the computer, then start up the computer.

Replacing memory in the top slot

After you've shut down your iMac and given it some time to cool down, follow these steps:

1. Disconnect the power cord and all other cables from your computer.
2. Place a soft, clean towel or cloth on the desk or other flat surface to prevent scratching the display.
3. Hold the sides of the computer and slowly lay the computer face-down on the towel or cloth.
4. Using a Phillips screwdriver, remove the RAM access door on the bottom of the iMac and set it aside:
   
5. Pull the two levers on each side of the memory compartment to eject the memory module that's already installed:
   
6. Remove the memory module from your iMac as shown below:
   
7. Insert your RAM SO-DIMM into the top slot, noting the orientation of the keyed SO-DIMM:
   
8. After you've inserted it, press the DIMM up into the slot with your thumbs. Don't use the DIMM ejector clips to push in the DIMM, as this may damage the SDRAM DIMM.There should be a slight click when you seat the memory fully.
9. Close the ejector clips:
   
10. Reinstall the memory access door:
    
11. Place the computer in its upright position. Reconnect the power cord and all other cables to the computer, then start up the computer.

If your iMac makes a tone after you install memory

iMac models introduced before 2017 might make a warning sound when you start up after installing or replacing memory:

• One tone, repeating every five seconds signals that no RAM is installed.
• Three successive tones, then a five-second pause (repeating) signals that RAM doesn't pass a data integrity check.

Dram Memory Slot (x2) Functioning

If you hear these tones, confirm that the memory you installed is compatible with your iMac and that it's installed correctly by reseating the memory. If your Mac continues to make the tone, contact Apple Support.

Confirm that your iMac recognises its new memory

After you've installed memory, you should confirm that your iMac recognises the new RAM by choosing Apple () menu > About This Mac.

The window that appears lists the total memory, including the amount of memory that originally came with the computer plus the newly added memory. If all the memory in the iMac has been replaced, it lists the new total of all installed RAM.

For detailed information about the memory installed in your iMac, click System Report. Then choose Memory under the Hardware section in the left side of System Information.

If your computer doesn't recognise the memory or doesn't start up correctly, confirm that the RAM you installed is compatible with your iMac and that it's installed correctly by reseating the memory. If you continue to have issues with the memory in your iMac, contact Apple Support.