diff --git a/4.md b/4.md
new file mode 100644
index 0000000..e965e6a
--- /dev/null
+++ b/4.md
@@ -0,0 +1,113 @@
+# Memory
+
+## Default Memory Access Permissions
+
+The Cortex-M3 memory map has a default configuration for memory access permissions
+Default memory access permission is used when either:
+* No MPU is present
+* MPU is present but disabled
+Otherwise MPU will determine
+When a memory access is blocked the fault exception takes place immediately.
+
+```python { cmd matplotlib hide }
+
+import pymupdf
+
+from PIL import Image
+
+doc = pymupdf.open("./pdf/L4.pdf")
+
+pix = doc[12].get_pixmap()
+img = Image.frombytes("RGB", [pix.width, pix.height], pix.samples)
+plt.axis('off')
+plt.subplots_adjust(left=0, right=1, top=1, bottom=0)
+plt.imshow(img)
+
+plt.show()
+
+```
+
+## MPU(Memory Protection Unit)
+
+MPU
+
+Regions can be overlapped only in ARMv7
+
+## Bit-Band Operations
+
+it allows a single load/store opertation to access a **single data bit**.
+
+Bit-band regions:
+* The first 1MB of the SRAM region
+* The first 1MB of theperipheral region
+
+They can be accessed via a separate memory region called the bit-band alias.
+
+Bit-Banding done transparently by **bus matrix**.
+
+### Write to Bit-Band Alias
+
+* Without Bit-Band:
+```armasm
+LDR   R0, =0x20000000 ; Setup address
+LDR   R1, [R0]        ; Read
+ORR.W R1, #0x4        ; Modify bit
+STR   R1, [R0]        ; Write back result
+```
+
+* With Bit-Band:
+```armasm
+LDR   R0, =0x22000008 ; Setup address
+MOV   R1, #1          ; Setup data
+STR   R1, [R0]        ; Write
+```
+
+
+### Read from the Bit-Band Alias
+
+To get bit 2 in word data in address `0x20000000`:
+* Without
+```armasm
+LDR    R0, =0x20000000 ; Setup address
+LDR    R1, [R0]        ; Read
+UBFX.W R1, R1, #2, #1  ; Extract Bit[2]
+```
+
+* With
+```armasm
+LDR R0, = 0x22000008 ; Setup address
+LDR R1, [R0]         ; Read
+```
+
+### Advantages of using bit-band ops
+
+* Faster bit operations with fewer instructions
+* Prevent a **race condition** problem in bit modification
+
+
+### Usecase in C
+
+* There is no native support of bit-band ops in most C compilers
+  * The simplest solution is to separately declare the address and the bit-band alias of a memory location
+
+```c
+#define DEVICE_REGO      *((volatile unsigned long *) 0x40000000)
+#define DEVICE REGO_BIT0 *((volatile unsigned long *) 0x42000000)
+#define DEVICE REGO_BIT1 *((volatile unsigned long *) 0x42000004)
+```
+
+
+## Unaligned Transfers
+
+Coretex-M3 optionally supports unaligned transfers on single accesses.
+Data memory accesses can be defined as aligned or unaligned.
+
+Unaligned transfers are converted into multiple aligned transfers by the processor's bus interface unit:
+* transparent to application programmers
+* it takes more clock cycles for a single data access
+
+Limitations:
+* Not supported in load/store multiple instructions
+* Stack operations (PUSH/POP) must be aligned
+* Exclusive access must be aligned
+* Unaligned transfers are not supported in bit-band ops
\ No newline at end of file
diff --git a/5.md b/5.md
new file mode 100644
index 0000000..e16c777
--- /dev/null
+++ b/5.md
@@ -0,0 +1,31 @@
+# ARM ISA
+
+## Syntax of Assembly
+
+* Label: Define a symbol
+```
+label:
+```
+
+* Instruction
+* Directive
+```
+.directive[;]
+```
+* macro_invocation
+
+## Assembly Expression
+
+Expressions consist of one or more integer literals or symbol references, combined using operators. Expression can be used as instruction operands or directive argument.
+
+Assembler evaluates all expression, which means **no rum-time evaluation**.
+
+Assembly Expressions are
+* Constnats
+* Symbol References
+* Operators
+  * Unary Operators: `+`, `-`, `~`
+  * Binary Operators: `+`, `-`, `*`, `/`, `%`
+  * Binary Logical Operators: `&&`, `||`
+  * Binary Bitwise Operators: `&`, `|`, `^`, `>>`, `<<`
+  * Binary Comparison Operators: `<`, `<=`, `>`, `>=`, `==`, `!=`
\ No newline at end of file
diff --git a/pdf/L4.pdf b/pdf/L4.pdf
new file mode 100644
index 0000000..eca84de
--- /dev/null
+++ b/pdf/L4.pdf
@@ -0,0 +1,3 @@
+version https://git-lfs.github.com/spec/v1
+oid sha256:c83bddb6f3ec48cc3941b0cb6d6e1fff18d763c5b592702fe73c932b26c536a8
+size 853485
diff --git a/pdf/L5.pdf b/pdf/L5.pdf
new file mode 100644
index 0000000..55e6601
--- /dev/null
+++ b/pdf/L5.pdf
@@ -0,0 +1,3 @@
+version https://git-lfs.github.com/spec/v1
+oid sha256:0962f248a9d48faf6c6fafebf73edbae636f026a5564075111253e70ac63ae53
+size 483414
diff --git a/pdf/L6.pdf b/pdf/L6.pdf
new file mode 100644
index 0000000..a7e2430
--- /dev/null
+++ b/pdf/L6.pdf
@@ -0,0 +1,3 @@
+version https://git-lfs.github.com/spec/v1
+oid sha256:25b6bfc90f30335dc080b0b20cb5f47d5f35fecde6c7d9ee227363ee61344ac2
+size 741285