Update README.md

Author: Robert Lindner

README.md

@@ -16,8 +16,8 @@ Commands:
 |:----------:|-------------|
 | LITERAL | Push next 4 bytes |
 | LITERAL_ARRAY | Get x from next 4 bytes; Push x sets of 4 bytes - temporary |
-| LOAD | Pop a; Push RAM[a] |
-| STORE | Pop b; Pop a; RAM[b] = a |
+| LOAD ; LOAD_ARG ; LOAD_LCL | Pop a; Push RAM[a] |
+| STORE ; STORE_LCL | Pop b; Pop a; RAM[b] = a |
 | ADD | Pop b; Pop a; Push a + b |
 | SUB | Pop b; Pop a; Push a - b |
 | LESS | Pop b; Pop a; Push a < b |
@@ -26,19 +26,38 @@ Commands:
 | EQUALS | Pop b; Pop a; Push a == b |
 | JMP | Pop a; goto a; |
 | JMP_IF | Pop b; Pop a; if a goto b |
+| CALL | put current state in a stack frame; store RTN; Pop a; goto a; |
+| RETURN | Restore to previous stack frame; append working stack; goto RTN |
 | PRINT | Pop x; for x Print Pop - temporary, will be a library function based on null terminated strings |
 | PRINT_INT | Pop a; Print string of a |
-| PRINT_ENDL | Start a new line in console
+| PRINT_ENDL | Start a new line in console |
+
+LOAD and STORE have segment modifiers that can be used as base addresses within functions
+
+| Segment | Description |
+|:----------:|-------------|
+| LCL | pointer to first local variable |
+| ARG | pointer to first argument |
+
+| Variable Type | Description |
+|:----------:|-------------|
+| static | a fixed address variable, referenced directly |
+| argument | a variable that sits in the working stack of the previous function's frame |
+| local | a local variable within the stack frame of the current function |
+
+There is no concept of scope so all variables should be unique
+Variables are declared implicitly upon their first occurance with the exception of arguments
+Variables declared before the first function are static, after the first function they are local
 
 The assembler also features Symbols (variables and labels)
  * Variables start with # and are statically allocated at compile time
  * Jump labels start with @
+ * Subroutines start with $ and are followed by argument declarations
 
 ### Planned
 
 I plan to add:
  * indexed library function support (executes function at pushed index)
- * Subroutines
  * Dynamic memory allocation
  * Support for standard types int float char bool (maybe short, long, double etc) unsigned or signed
  * Built in support for variable length arrays, strings and vectors