Add more instructions to the assembler

Includes:
 - JMP
 - RET
 - JSR
 - JSRR
 - LDI
 - LEA
 - ST
 - STR
 - STI

Includes some more bug fixes and error handling:

 - return SymbolError from SymbolTable.Offset so it can be unwrapped.
 - code docs updated
 - types cleanup

Squashed commit of 7d10b6b..8c5cca9

TODO.md

@@ -1,32 +1,21 @@
 # TODO #
 
-- [ ] DOCS:
-  - [ ] tutorial
-  - [ ] design
-- [.] ASM:
-  - [.] code generation: ~7/16
-  - [.] directives
+- ASM:
+  - [.] document grammar more completely
+  - directives remaining:
     - [ ] .END
     - [ ] .EXTERNAL
-    - [x] .STRINGZ: strangz
-    - [x] .BLKW: block words
-    - [x] .DW: define word
-    - [x] .FILL: fill word
-    - [x] .ORIG: origin
-  - [x] cli command
-  - [.] document grammar more completely
-  - [x] memory layout
-  - [x] cleaner error handling
-  - [x] simple parser: regexp
-  - [x] symbol table
 - [ ] LOAD: object loader
+- DOCS:
+  - [ ] tutorial
+  - [ ] design
+- [ ] MONITOR: trap, exception, and interrupt routines
 - [ ] LINK: code linker
 - [ ] DUMP: hex encoder
 - [ ] CLI: sub commands for vm, tools, terminal, shell
 - [ ] LOG: program output to STDOUT, logging output to STDERR (unless in
       demo)
 - [ ] TERM: finish the terminal I/O
-- [ ] MONITOR: trap, exception, and interrupt routines
 - [ ] REPL: step debugger shell
 - [ ] TIMER: simple timer device
 - [ ] NEWLANG: interpreted language; threaded compiler
@@ -35,9 +24,23 @@
 
 ## DID ##
 
-- [x] instruction loop
-- [x] instructions
-- [x] service exceptions
-- [x] service traps
-- [x] memory mapped I/O
-- [x] hardware interrupts
+- ASM:
+  - [x] code generation
+  - [x] completed directives:
+    - [x] .STRINGZ: strangz
+    - [x] .BLKW: block words
+    - [x] .DW: define word
+    - [x] .FILL: fill word
+    - [x] .ORIG: origin
+  - [x] cli command
+  - [x] memory layout
+  - [x] cleaner error handling
+  - [x] simple parser: regexp
+  - [x] symbol table
+- VM:
+  - [x] instruction loop
+  - [x] instructions
+  - [x] service exceptions
+  - [x] service traps
+  - [x] memory mapped I/O
+  - [x] hardware interrupts

internal/asm/asm.go

@@ -92,7 +92,7 @@ func (s SymbolTable) Add(sym string, loc uint16) {
 func (s SymbolTable) Offset(sym string, pc uint16, n int) (uint16, error) {
 	loc, ok := s[sym]
 	if !ok {
-		return 0xffff, fmt.Errorf("%s: symbol not found", sym)
+		return 0xffff, &SymbolError{Symbol: sym, Loc: pc}
 	}
 
 	delta := int16(loc - pc)
@@ -155,6 +155,14 @@ func (se *SymbolError) Error() string {
 	return fmt.Sprintf("symbol error: %q", se.Symbol)
 }
 
+func (se *SymbolError) Is(err error) bool {
+	if _, ok := err.(*SymbolError); ok {
+		return true
+	}
+
+	return false
+}
+
 // SyntaxTable is holds the parsed code and data indexed by its location counter.
 type SyntaxTable []Operation