Add information on Program Segments for PPC EABI programs

Author: JoshSanch

src/asm-concepts/main.typ

@@ -1,13 +1,47 @@
 = Assembly Concepts
 
-== Sections
-=== .comm
-=== .bss
-=== .ctors
+== Program Segments
+A code segment (and its companion, the data segment) is how a compiler and linker organize a program's contents into named regions. 
+.text holds executable instructions, .rodata holds immutable constants like string literals, .data holds initialized writable globals, 
+and .bss/.comm reserve space for uninitialized globals without storing bytes in the object file. 
+
+Special small-data sections (.sdata, .sbss, .sdata2, .sbss2) group frequently accessed or size‑limited globals to enable more 
+efficient addressing on some ABIs (for example, keeping them within a 16‑bit offset of a base register). 
+
+These sections let the linker combine object files, resolve symbol addresses, and produce an executable image with
+correct memory layout and access permissions, which the loader then maps into memory at runtime.
+
+What follows is a dedicated description of each type of program segment we will encounter in a program
+compiled to target the PowerPC EABI architecture.
+
+=== .comm (Common)
+Contains uninitialized global variables. These symbols are shared across object files during linking.
+
+=== .bss (Block Started By Symbol)
+Contains uninitialized static variables and global variables. Takes no space in the object file but reserves space in memory when loaded.
+
+=== .ctors ("Constructors")
+Contains "constructor" function pointers that are executed before main(). Used for global object initialization in C++.
+Note that these functions are *not* class/struct constructors, but are simply functions called prior to the program entry point being called,
+which is typically the first function that is called.
+
 === .data
-=== .rodata
-=== .sbss
-=== .sbss2
-=== .sdata
+Contains initialized static and global variables that can be modified during program execution.
+
+=== .rodata (Read-Only Data)
+Contains constant values like string literals and const variables that shouldn't be modified during execution.
+
+=== .sbss (Small BSS)
+Contains uninitialized static/global variables that are 8 bytes or smaller.
+
+=== .sbss2 (Small BSS 2)
+Contains static/global variables of size 8 bytes or smaller which are initialized to zero.
+
+=== .sdata (Small Data)
+Contains initialized static/global variables of size 8 bytes or smaller. .sdata will contain values that are explicitly non-zero.
+
 === .sdata2
-=== .text
+Contains initialized static/global variables of size 8 bytes or smaller optimized for read-only access, although it can contain writablev values.
+
+=== .text
+Contains the actual executable code (machine instructions) of the program. This section is typically read-only to prevent code modification during execution.