modify README.md

README.md

@@ -1,10 +1,46 @@
 # 8-Bit CPU Emulator
 
-## CPU Architecture
-- Word Size
-    - **Data Width:** 8 bits
-    - **Address width:** 16 bits
-    - **Address space:** 64 KB (0x0000-0xFFFF)
+This project is an educational 8-bit CPU emulator written in Rust, it's build to understand core computer architecture concepts such as instruction sets, flags,
+control flow, stacks and system calls.
+
+This project is **not** a production CPU and **not** accurate. It is designed purely for learning and experimentation.
+
+-----
+
+## Features
+
+- Custom 8-bit ISA (control-flow and arithmetic operations)
+- 4 general-purpose registers
+- 64 KB (0x0000-0xFFFF) byte-addressable memory
+- Two-pass assembler with label support
+- Stack-based function calls
+-  Stack is software-managed and grows downward
+
+---
+
+
+
+## Registers
+| Register | Size   | Description                    |
+| -------- | ------ | ------------------------------ |
+| A        | 8-bit  | General-purpose register                        |
+| B        | 8-bit  | General-purpose register                        |
+| C        | 8-bit  | General-purpose register                        |
+| D        | 8-bit  | General-purpose register                        |
+| PC       | 16-bit | Program Counter                |
+| SP       | 16-bit | Stack pointer                  |
+
+
+## Flags Register
+
+The CPU maintains a small flags register.
+
+| Flag  | Description  |
+| ----- | ------------ |
+| Z     | Zero flag - set if last result was `0`   |
+| C     | Carry/Borrow flag |
+
+- Control-flow instructions do **not** modify flags.
 
 ## Supported Instructions
 
@@ -21,41 +57,48 @@
 | DIV         | div r1, r2     |
 | CALL        | call \<label\>     |
 | SYS         | sys \<syscall_no\> |
-| HLT (Halt)  | hlt              |
-
-
-## Registers
-| Register | Size   | Description                    |
-| -------- | ------ | ------------------------------ |
-| A        | 8-bit  | General                        |
-| B        | 8-bit  | General                        |
-| C        | 8-bit  | General                        |
-| D        | 8-bit  | General                        |
-| PC       | 16-bit | Program Counter                |
-| SP       | 16-bit | Stack pointer                  |
-
-
-## Flags
-| Flag  | Description  |
-| ----- | ------------ |
-| Z     | Zero Flag    |
-| C     | Carry/Borrow |
-
+| HLT (Halt)  | hlt                                |
 
 ## Syscalls
+
 | Imm | Meaning |
 | --- | ------- |
 | 0   | Exit Program |
 | 1   | Print register A as integer |
 | 2   | Print register A as ASCII char |
 
+- Syscalls also do not modify flags
+
+## Example assembly program
+
+```assembly
+mov b, 3
+mov a, 1
+
+loop:
+    sub b, a
+    jnz loop
+
+sys 1   ; print register A
+sys 0   ; exit
+```
+
+## Usage
 
-# Usage
 ```bash
 cargo run -- --f <examples/filename.asc>
 ```
 
-## Todo
+## Non-goals
+
+1. No pipelining
+2. No interrupts
+3. No virtual memory
+4. No privilege levels
+5. No hardware I/O
+
+## Goals
+
 - [x] Assembler
     - [x] Lexer/Tokenizer
     - [x] Add label support (supporting JMP/JZ/JNZ)
@@ -66,5 +109,5 @@ cargo run -- --f <examples/filename.asc>
     - [x] CALL
     - [x] RET
     - [x] SYS
-- [ ] Better Error handling
-- [ ] TUI Debugger
+- [ ] Better error-handling
+- [ ] TUI debugger