diff --git a/.zk/notebook.db b/.zk/notebook.db
index 1a0a92f..dc05620 100644
Binary files a/.zk/notebook.db and b/.zk/notebook.db differ
diff --git a/zk/DRAM_and_SRAM_memory.md b/zk/DRAM_and_SRAM_memory.md
new file mode 100644
index 0000000..30d076e
--- /dev/null
+++ b/zk/DRAM_and_SRAM_memory.md
@@ -0,0 +1,9 @@
+---
+title: DRAM_and_SRAM_memory
+tags: [memory]
+created: Friday, July 12, 2024
+---
+
+# DRAM_and_SRAM_memory
+
+## Related notes
diff --git a/zk/Memory_addresses.md b/zk/Memory_addresses.md
index 2251830..5b446b7 100644
--- a/zk/Memory_addresses.md
+++ b/zk/Memory_addresses.md
@@ -46,12 +46,12 @@ is the number of bits.
 We need to reverse this formula to find out how many bits we need to represent a
 given number of addresses. We can do this with a [logarithm](Logarithms.md).
 
-We can reverse the formula as follows: number of bits = $\log_2$(number of
+We can reverse the formula as follows: number of bits = $\log_2$ (number of
 addresses).
 
 In our case we have 65,536 addresses so we need $\log_2(65,536)$ bits to
 represent each address. This is approximately 16 bits. Thus a 16 bit memory
-address is needed to address 65, 546 bytes.
+iaddress is needed to address 65, 546 bytes.
 
 Using memory addresses we end up with tables like the following:
 
diff --git a/zk/Reducing_fractions.md b/zk/Reducing_fractions.md
index 2fbf00e..427253c 100644
--- a/zk/Reducing_fractions.md
+++ b/zk/Reducing_fractions.md
@@ -42,7 +42,7 @@ With this method, the reduction can be completed in a single step.
 The greatest common divisor of 18 and 24 is 6, thus:
 
 $$
-\\frac{18}{24} = \frac{18/6}{24/6} = \frac{3}{4}
+\frac{18}{24} = \frac{18/6}{24/6} = \frac{3}{4}
 $$
 
 Note how our earlier two divisors 2 and 3 are
diff --git a/zk/VirtualMemory.md b/zk/VirtualMemory.md
index 2993e42..704ccd4 100644
--- a/zk/VirtualMemory.md
+++ b/zk/VirtualMemory.md
@@ -52,4 +52,4 @@ other with separate address spaces that cannot interact.
 // Next: the kernel also uses virtual memory however isn't also responsible for
 the appportioning of virtual memory. Confused.
 
-![](/img/virtual-memory-diagram.jpg)
+![Virtual memory diagram](/img/virtual-memory-diagram.jpg)
diff --git a/zk/What_is_memory.md b/zk/What_is_memory.md
new file mode 100644
index 0000000..f5ad72d
--- /dev/null
+++ b/zk/What_is_memory.md
@@ -0,0 +1,35 @@
+---
+title: What_is_memory
+tags: [memory]
+created: Friday, July 12, 2024
+---
+
+# What is memory ?
+
+## Why do we need memory?
+
+> A CPU is just an operator on memory. It reads its instructions and data from
+> the memory and writes back out to the memory. (Ward 2021)
+
+When a [CPU](CPU_architecture.md) executes a program, it needs a place to store
+the program's **instructions** and **related data**. This is the role of memory.
+
+## What is memory?
+
+The data that comprises a program is a series of bits. The basic unit of memory
+storage is a **memory cell**: a circuit that can store a single bit.
+
+## Memory types
+
+There are two types of memory: [SRAM and DRAM](./DRAM_and_SRAM_memory.md). Both
+types of RAM memory are _volatile_ : the memory is only retained whilst the
+computer has a power supply and is wiped when the computer is rebooted. This
+contrasts with the memory of the harddisk which is non-volatile and is retained
+after a reboot.
+
+Programs that are executing are loaded into memory because the chips that
+comprise memory can read and store data much faster than the harddisk. It would
+be possible to run a program from the harddisk but it would be 500 - 1000 times
+slower than memory.
+
+## Related notes