diff --git a/LICENSE b/LICENSE
new file mode 100644
index 0000000..9efa6fb
--- /dev/null
+++ b/LICENSE
@@ -0,0 +1,338 @@
+                    GNU GENERAL PUBLIC LICENSE
+                       Version 2, June 1991
+
+ Copyright (C) 1989, 1991 Free Software Foundation, Inc.,
+ <https://fsf.org/>
+ Everyone is permitted to copy and distribute verbatim copies
+ of this license document, but changing it is not allowed.
+
+                            Preamble
+
+  The licenses for most software are designed to take away your
+freedom to share and change it.  By contrast, the GNU General Public
+License is intended to guarantee your freedom to share and change free
+software--to make sure the software is free for all its users.  This
+General Public License applies to most of the Free Software
+Foundation's software and to any other program whose authors commit to
+using it.  (Some other Free Software Foundation software is covered by
+the GNU Lesser General Public License instead.)  You can apply it to
+your programs, too.
+
+  When we speak of free software, we are referring to freedom, not
+price.  Our General Public Licenses are designed to make sure that you
+have the freedom to distribute copies of free software (and charge for
+this service if you wish), that you receive source code or can get it
+if you want it, that you can change the software or use pieces of it
+in new free programs; and that you know you can do these things.
+
+  To protect your rights, we need to make restrictions that forbid
+anyone to deny you these rights or to ask you to surrender the rights.
+These restrictions translate to certain responsibilities for you if you
+distribute copies of the software, or if you modify it.
+
+  For example, if you distribute copies of such a program, whether
+gratis or for a fee, you must give the recipients all the rights that
+you have.  You must make sure that they, too, receive or can get the
+source code.  And you must show them these terms so they know their
+rights.
+
+  We protect your rights with two steps: (1) copyright the software, and
+(2) offer you this license which gives you legal permission to copy,
+distribute and/or modify the software.
+
+  Also, for each author's protection and ours, we want to make certain
+that everyone understands that there is no warranty for this free
+software.  If the software is modified by someone else and passed on, we
+want its recipients to know that what they have is not the original, so
+that any problems introduced by others will not reflect on the original
+authors' reputations.
+
+  Finally, any free program is threatened constantly by software
+patents.  We wish to avoid the danger that redistributors of a free
+program will individually obtain patent licenses, in effect making the
+program proprietary.  To prevent this, we have made it clear that any
+patent must be licensed for everyone's free use or not licensed at all.
+
+  The precise terms and conditions for copying, distribution and
+modification follow.
+
+                    GNU GENERAL PUBLIC LICENSE
+   TERMS AND CONDITIONS FOR COPYING, DISTRIBUTION AND MODIFICATION
+
+  0. This License applies to any program or other work which contains
+a notice placed by the copyright holder saying it may be distributed
+under the terms of this General Public License.  The "Program", below,
+refers to any such program or work, and a "work based on the Program"
+means either the Program or any derivative work under copyright law:
+that is to say, a work containing the Program or a portion of it,
+either verbatim or with modifications and/or translated into another
+language.  (Hereinafter, translation is included without limitation in
+the term "modification".)  Each licensee is addressed as "you".
+
+Activities other than copying, distribution and modification are not
+covered by this License; they are outside its scope.  The act of
+running the Program is not restricted, and the output from the Program
+is covered only if its contents constitute a work based on the
+Program (independent of having been made by running the Program).
+Whether that is true depends on what the Program does.
+
+  1. You may copy and distribute verbatim copies of the Program's
+source code as you receive it, in any medium, provided that you
+conspicuously and appropriately publish on each copy an appropriate
+copyright notice and disclaimer of warranty; keep intact all the
+notices that refer to this License and to the absence of any warranty;
+and give any other recipients of the Program a copy of this License
+along with the Program.
+
+You may charge a fee for the physical act of transferring a copy, and
+you may at your option offer warranty protection in exchange for a fee.
+
+  2. You may modify your copy or copies of the Program or any portion
+of it, thus forming a work based on the Program, and copy and
+distribute such modifications or work under the terms of Section 1
+above, provided that you also meet all of these conditions:
+
+    a) You must cause the modified files to carry prominent notices
+    stating that you changed the files and the date of any change.
+
+    b) You must cause any work that you distribute or publish, that in
+    whole or in part contains or is derived from the Program or any
+    part thereof, to be licensed as a whole at no charge to all third
+    parties under the terms of this License.
+
+    c) If the modified program normally reads commands interactively
+    when run, you must cause it, when started running for such
+    interactive use in the most ordinary way, to print or display an
+    announcement including an appropriate copyright notice and a
+    notice that there is no warranty (or else, saying that you provide
+    a warranty) and that users may redistribute the program under
+    these conditions, and telling the user how to view a copy of this
+    License.  (Exception: if the Program itself is interactive but
+    does not normally print such an announcement, your work based on
+    the Program is not required to print an announcement.)
+
+These requirements apply to the modified work as a whole.  If
+identifiable sections of that work are not derived from the Program,
+and can be reasonably considered independent and separate works in
+themselves, then this License, and its terms, do not apply to those
+sections when you distribute them as separate works.  But when you
+distribute the same sections as part of a whole which is a work based
+on the Program, the distribution of the whole must be on the terms of
+this License, whose permissions for other licensees extend to the
+entire whole, and thus to each and every part regardless of who wrote it.
+
+Thus, it is not the intent of this section to claim rights or contest
+your rights to work written entirely by you; rather, the intent is to
+exercise the right to control the distribution of derivative or
+collective works based on the Program.
+
+In addition, mere aggregation of another work not based on the Program
+with the Program (or with a work based on the Program) on a volume of
+a storage or distribution medium does not bring the other work under
+the scope of this License.
+
+  3. You may copy and distribute the Program (or a work based on it,
+under Section 2) in object code or executable form under the terms of
+Sections 1 and 2 above provided that you also do one of the following:
+
+    a) Accompany it with the complete corresponding machine-readable
+    source code, which must be distributed under the terms of Sections
+    1 and 2 above on a medium customarily used for software interchange; or,
+
+    b) Accompany it with a written offer, valid for at least three
+    years, to give any third party, for a charge no more than your
+    cost of physically performing source distribution, a complete
+    machine-readable copy of the corresponding source code, to be
+    distributed under the terms of Sections 1 and 2 above on a medium
+    customarily used for software interchange; or,
+
+    c) Accompany it with the information you received as to the offer
+    to distribute corresponding source code.  (This alternative is
+    allowed only for noncommercial distribution and only if you
+    received the program in object code or executable form with such
+    an offer, in accord with Subsection b above.)
+
+The source code for a work means the preferred form of the work for
+making modifications to it.  For an executable work, complete source
+code means all the source code for all modules it contains, plus any
+associated interface definition files, plus the scripts used to
+control compilation and installation of the executable.  However, as a
+special exception, the source code distributed need not include
+anything that is normally distributed (in either source or binary
+form) with the major components (compiler, kernel, and so on) of the
+operating system on which the executable runs, unless that component
+itself accompanies the executable.
+
+If distribution of executable or object code is made by offering
+access to copy from a designated place, then offering equivalent
+access to copy the source code from the same place counts as
+distribution of the source code, even though third parties are not
+compelled to copy the source along with the object code.
+
+  4. You may not copy, modify, sublicense, or distribute the Program
+except as expressly provided under this License.  Any attempt
+otherwise to copy, modify, sublicense or distribute the Program is
+void, and will automatically terminate your rights under this License.
+However, parties who have received copies, or rights, from you under
+this License will not have their licenses terminated so long as such
+parties remain in full compliance.
+
+  5. You are not required to accept this License, since you have not
+signed it.  However, nothing else grants you permission to modify or
+distribute the Program or its derivative works.  These actions are
+prohibited by law if you do not accept this License.  Therefore, by
+modifying or distributing the Program (or any work based on the
+Program), you indicate your acceptance of this License to do so, and
+all its terms and conditions for copying, distributing or modifying
+the Program or works based on it.
+
+  6. Each time you redistribute the Program (or any work based on the
+Program), the recipient automatically receives a license from the
+original licensor to copy, distribute or modify the Program subject to
+these terms and conditions.  You may not impose any further
+restrictions on the recipients' exercise of the rights granted herein.
+You are not responsible for enforcing compliance by third parties to
+this License.
+
+  7. If, as a consequence of a court judgment or allegation of patent
+infringement or for any other reason (not limited to patent issues),
+conditions are imposed on you (whether by court order, agreement or
+otherwise) that contradict the conditions of this License, they do not
+excuse you from the conditions of this License.  If you cannot
+distribute so as to satisfy simultaneously your obligations under this
+License and any other pertinent obligations, then as a consequence you
+may not distribute the Program at all.  For example, if a patent
+license would not permit royalty-free redistribution of the Program by
+all those who receive copies directly or indirectly through you, then
+the only way you could satisfy both it and this License would be to
+refrain entirely from distribution of the Program.
+
+If any portion of this section is held invalid or unenforceable under
+any particular circumstance, the balance of the section is intended to
+apply and the section as a whole is intended to apply in other
+circumstances.
+
+It is not the purpose of this section to induce you to infringe any
+patents or other property right claims or to contest validity of any
+such claims; this section has the sole purpose of protecting the
+integrity of the free software distribution system, which is
+implemented by public license practices.  Many people have made
+generous contributions to the wide range of software distributed
+through that system in reliance on consistent application of that
+system; it is up to the author/donor to decide if he or she is willing
+to distribute software through any other system and a licensee cannot
+impose that choice.
+
+This section is intended to make thoroughly clear what is believed to
+be a consequence of the rest of this License.
+
+  8. If the distribution and/or use of the Program is restricted in
+certain countries either by patents or by copyrighted interfaces, the
+original copyright holder who places the Program under this License
+may add an explicit geographical distribution limitation excluding
+those countries, so that distribution is permitted only in or among
+countries not thus excluded.  In such case, this License incorporates
+the limitation as if written in the body of this License.
+
+  9. The Free Software Foundation may publish revised and/or new versions
+of the General Public License from time to time.  Such new versions will
+be similar in spirit to the present version, but may differ in detail to
+address new problems or concerns.
+
+Each version is given a distinguishing version number.  If the Program
+specifies a version number of this License which applies to it and "any
+later version", you have the option of following the terms and conditions
+either of that version or of any later version published by the Free
+Software Foundation.  If the Program does not specify a version number of
+this License, you may choose any version ever published by the Free Software
+Foundation.
+
+  10. If you wish to incorporate parts of the Program into other free
+programs whose distribution conditions are different, write to the author
+to ask for permission.  For software which is copyrighted by the Free
+Software Foundation, write to the Free Software Foundation; we sometimes
+make exceptions for this.  Our decision will be guided by the two goals
+of preserving the free status of all derivatives of our free software and
+of promoting the sharing and reuse of software generally.
+
+                            NO WARRANTY
+
+  11. BECAUSE THE PROGRAM IS LICENSED FREE OF CHARGE, THERE IS NO WARRANTY
+FOR THE PROGRAM, TO THE EXTENT PERMITTED BY APPLICABLE LAW.  EXCEPT WHEN
+OTHERWISE STATED IN WRITING THE COPYRIGHT HOLDERS AND/OR OTHER PARTIES
+PROVIDE THE PROGRAM "AS IS" WITHOUT WARRANTY OF ANY KIND, EITHER EXPRESSED
+OR IMPLIED, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF
+MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE.  THE ENTIRE RISK AS
+TO THE QUALITY AND PERFORMANCE OF THE PROGRAM IS WITH YOU.  SHOULD THE
+PROGRAM PROVE DEFECTIVE, YOU ASSUME THE COST OF ALL NECESSARY SERVICING,
+REPAIR OR CORRECTION.
+
+  12. IN NO EVENT UNLESS REQUIRED BY APPLICABLE LAW OR AGREED TO IN WRITING
+WILL ANY COPYRIGHT HOLDER, OR ANY OTHER PARTY WHO MAY MODIFY AND/OR
+REDISTRIBUTE THE PROGRAM AS PERMITTED ABOVE, BE LIABLE TO YOU FOR DAMAGES,
+INCLUDING ANY GENERAL, SPECIAL, INCIDENTAL OR CONSEQUENTIAL DAMAGES ARISING
+OUT OF THE USE OR INABILITY TO USE THE PROGRAM (INCLUDING BUT NOT LIMITED
+TO LOSS OF DATA OR DATA BEING RENDERED INACCURATE OR LOSSES SUSTAINED BY
+YOU OR THIRD PARTIES OR A FAILURE OF THE PROGRAM TO OPERATE WITH ANY OTHER
+PROGRAMS), EVEN IF SUCH HOLDER OR OTHER PARTY HAS BEEN ADVISED OF THE
+POSSIBILITY OF SUCH DAMAGES.
+
+                     END OF TERMS AND CONDITIONS
+
+            How to Apply These Terms to Your New Programs
+
+  If you develop a new program, and you want it to be of the greatest
+possible use to the public, the best way to achieve this is to make it
+free software which everyone can redistribute and change under these terms.
+
+  To do so, attach the following notices to the program.  It is safest
+to attach them to the start of each source file to most effectively
+convey the exclusion of warranty; and each file should have at least
+the "copyright" line and a pointer to where the full notice is found.
+
+    <one line to give the program's name and a brief idea of what it does.>
+    Copyright (C) <year>  <name of author>
+
+    This program is free software; you can redistribute it and/or modify
+    it under the terms of the GNU General Public License as published by
+    the Free Software Foundation; either version 2 of the License, or
+    (at your option) any later version.
+
+    This program is distributed in the hope that it will be useful,
+    but WITHOUT ANY WARRANTY; without even the implied warranty of
+    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+    GNU General Public License for more details.
+
+    You should have received a copy of the GNU General Public License along
+    with this program; if not, see <https://www.gnu.org/licenses/>.
+
+Also add information on how to contact you by electronic and paper mail.
+
+If the program is interactive, make it output a short notice like this
+when it starts in an interactive mode:
+
+    Gnomovision version 69, Copyright (C) year name of author
+    Gnomovision comes with ABSOLUTELY NO WARRANTY; for details type `show w'.
+    This is free software, and you are welcome to redistribute it
+    under certain conditions; type `show c' for details.
+
+The hypothetical commands `show w' and `show c' should show the appropriate
+parts of the General Public License.  Of course, the commands you use may
+be called something other than `show w' and `show c'; they could even be
+mouse-clicks or menu items--whatever suits your program.
+
+You should also get your employer (if you work as a programmer) or your
+school, if any, to sign a "copyright disclaimer" for the program, if
+necessary.  Here is a sample; alter the names:
+
+  Yoyodyne, Inc., hereby disclaims all copyright interest in the program
+  `Gnomovision' (which makes passes at compilers) written by James Hacker.
+
+  <signature of Moe Ghoul>, 1 April 1989
+  Moe Ghoul, President of Vice
+
+This General Public License does not permit incorporating your program into
+proprietary programs.  If your program is a subroutine library, you may
+consider it more useful to permit linking proprietary applications with the
+library.  If this is what you want to do, use the GNU Lesser General
+Public License instead of this License.
diff --git a/main_test.go b/main_test.go
new file mode 100644
index 0000000..08a5cd9
--- /dev/null
+++ b/main_test.go
@@ -0,0 +1,541 @@
+package main
+
+import (
+	"fmt"
+	"path/filepath"
+	"strings"
+	"testing"
+)
+
+// TestParseArgs tests the flexible argument parsing logic
+func TestParseArgs(t *testing.T) {
+	tests := []struct {
+		name           string
+		args           []string
+		wantInput      string
+		wantOutput     string
+		wantIsBuild    bool
+		wantKeepAsm    bool
+		wantNoCBM      bool
+		expectError    bool
+	}{
+		// Simple file argument (default build mode)
+		{
+			name:        "simple file argument",
+			args:        []string{"program.c65"},
+			wantInput:   "program.c65",
+			wantOutput:  "program.prg",
+			wantIsBuild: true,
+		},
+		// Short -i flag
+		{
+			name:        "short -i flag",
+			args:        []string{"-i", "program.c65"},
+			wantInput:   "program.c65",
+			wantOutput:  "program.prg",
+			wantIsBuild: true,
+		},
+		// Long -in flag (legacy)
+		{
+			name:        "long -in flag",
+			args:        []string{"-in", "program.c65"},
+			wantInput:   "program.c65",
+			wantOutput:  "program.prg",
+			wantIsBuild: true,
+		},
+		// With output .prg (build mode)
+		{
+			name:        "with output .prg",
+			args:        []string{"program.c65", "-o", "game.prg"},
+			wantInput:   "program.c65",
+			wantOutput:  "game.prg",
+			wantIsBuild: true,
+		},
+		// With output .asm (compile mode)
+		{
+			name:        "with output .asm",
+			args:        []string{"program.c65", "-o", "output.asm"},
+			wantInput:   "program.c65",
+			wantOutput:  "output.asm",
+			wantIsBuild: false,
+		},
+		// With -out flag (legacy)
+		{
+			name:        "with -out flag",
+			args:        []string{"program.c65", "-out", "output.asm"},
+			wantInput:   "program.c65",
+			wantOutput:  "output.asm",
+			wantIsBuild: false,
+		},
+		// With keep-asm flag
+		{
+			name:        "with keep-asm flag",
+			args:        []string{"program.c65", "--keep-asm"},
+			wantInput:   "program.c65",
+			wantOutput:  "program.prg",
+			wantIsBuild: true,
+			wantKeepAsm: true,
+		},
+		// With no-cbm flag
+		{
+			name:        "with no-cbm flag",
+			args:        []string{"program.c65", "--no-cbm"},
+			wantInput:   "program.c65",
+			wantOutput:  "program.prg",
+			wantIsBuild: true,
+			wantNoCBM:   true,
+		},
+		// Mixed flags
+		{
+			name:        "mixed flags",
+			args:        []string{"-i", "program.c65", "-o", "out.prg", "--keep-asm", "--no-cbm"},
+			wantInput:   "program.c65",
+			wantOutput:  "out.prg",
+			wantIsBuild: true,
+			wantKeepAsm: true,
+			wantNoCBM:   true,
+		},
+		// No input file (error)
+		{
+			name:        "no input file",
+			args:        []string{"-o", "output.prg"},
+			expectError: true,
+		},
+		// Empty args (error)
+		{
+			name:        "empty args",
+			args:        []string{},
+			expectError: true,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			input, output, isBuild, keepAsm, noCBM, err := parseArgs(tt.args)
+			
+			if tt.expectError {
+				if err == nil {
+					t.Errorf("parseArgs() expected error, got nil")
+				}
+				return
+			}
+			
+			if err != nil {
+				t.Errorf("parseArgs() unexpected error: %v", err)
+				return
+			}
+			
+			if input != tt.wantInput {
+				t.Errorf("parseArgs() input = %v, want %v", input, tt.wantInput)
+			}
+			
+			if output != tt.wantOutput {
+				t.Errorf("parseArgs() output = %v, want %v", output, tt.wantOutput)
+			}
+			
+			if isBuild != tt.wantIsBuild {
+				t.Errorf("parseArgs() isBuild = %v, want %v", isBuild, tt.wantIsBuild)
+			}
+			
+			if keepAsm != tt.wantKeepAsm {
+				t.Errorf("parseArgs() keepAsm = %v, want %v", keepAsm, tt.wantKeepAsm)
+			}
+			
+			if noCBM != tt.wantNoCBM {
+				t.Errorf("parseArgs() noCBM = %v, want %v", noCBM, tt.wantNoCBM)
+			}
+		})
+	}
+}
+
+// TestOutputExtensionDetection tests detection of build vs compile mode by output extension
+func TestOutputExtensionDetection(t *testing.T) {
+	tests := []struct {
+		name       string
+		outputFile string
+		wantIsBuild bool
+	}{
+		{".prg extension", "program.prg", true},
+		{".PRG uppercase", "program.PRG", true},
+		{".asm extension", "program.asm", false},
+		{".ASM uppercase", "program.ASM", false},
+		{".s extension", "program.s", false},
+		{"no extension", "program", false}, // No .prg extension = compile mode
+		{".bin extension", "program.bin", false}, // No .prg extension = compile mode
+	}
+	
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			isBuild := strings.HasSuffix(strings.ToLower(tt.outputFile), ".prg")
+			if isBuild != tt.wantIsBuild {
+				t.Errorf("isBuild(%q) = %v, want %v", tt.outputFile, isBuild, tt.wantIsBuild)
+			}
+		})
+	}
+}
+
+// TestDefaultOutputNaming tests default output filename generation
+func TestDefaultOutputNaming(t *testing.T) {
+	tests := []struct {
+		name      string
+		inputFile string
+		isBuild   bool
+		wantOutput string
+	}{
+		{"build mode .c65", "program.c65", true, "program.prg"},
+		{"compile mode .c65", "program.c65", false, "program.asm"},
+		{"build mode .C65 uppercase", "program.C65", true, "program.prg"},
+		{"no extension", "program", true, "program.prg"},
+		{"multiple dots", "my.program.c65", true, "my.program.prg"},
+		{"path with extension", "/path/to/program.c65", true, "program.prg"},
+	}
+	
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			base := strings.TrimSuffix(filepath.Base(tt.inputFile), filepath.Ext(tt.inputFile))
+			var output string
+			if tt.isBuild {
+				output = base + ".prg"
+			} else {
+				output = base + ".asm"
+			}
+			
+			if output != tt.wantOutput {
+				t.Errorf("defaultOutput(%q, %v) = %v, want %v", tt.inputFile, tt.isBuild, output, tt.wantOutput)
+			}
+		})
+	}
+}
+
+// TestSubcommandParsing tests explicit subcommand parsing
+func TestSubcommandParsing(t *testing.T) {
+	tests := []struct {
+		name        string
+		args        []string
+		wantCommand string
+		expectError bool
+	}{
+		{"build command", []string{"build", "-i", "program.c65"}, "build", false},
+		{"compile command", []string{"compile", "-i", "program.c65"}, "compile", false},
+		{"help command", []string{"help"}, "help", false},
+		{"unknown command", []string{"unknown", "program.c65"}, "", true},
+	}
+	
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if len(tt.args) == 0 {
+				t.Skip("empty args")
+			}
+			
+			cmd := tt.args[0]
+			isSubcommand := cmd == "build" || cmd == "compile" || cmd == "help"
+			
+			if tt.expectError && isSubcommand {
+				t.Errorf("Expected error for command %q but it's a valid subcommand", cmd)
+			}
+			
+			if !tt.expectError && cmd != tt.wantCommand {
+				t.Errorf("Command = %q, want %q", cmd, tt.wantCommand)
+			}
+		})
+	}
+}
+
+// TestACMECommandBuilding tests building the ACME command line arguments
+func TestACMECommandBuilding(t *testing.T) {
+	tests := []struct {
+		name     string
+		output   string
+		asmFile  string
+		noCBM    bool
+		wantArgs []string
+	}{
+		{
+			name:     "default with cbm",
+			output:   "program.prg",
+			asmFile:  "temp.asm",
+			noCBM:    false,
+			wantArgs: []string{"-o", "program.prg", "-f", "cbm", "temp.asm"},
+		},
+		{
+			name:     "with no-cbm flag",
+			output:   "program.prg",
+			asmFile:  "temp.asm",
+			noCBM:    true,
+			wantArgs: []string{"-o", "program.prg", "temp.asm"},
+		},
+		{
+			name:     "different output name",
+			output:   "game.prg",
+			asmFile:  "temp.asm",
+			noCBM:    false,
+			wantArgs: []string{"-o", "game.prg", "-f", "cbm", "temp.asm"},
+		},
+	}
+	
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			args := buildACMEArgs(tt.output, tt.asmFile, tt.noCBM)
+			
+			if len(args) != len(tt.wantArgs) {
+				t.Errorf("buildACMEArgs() length = %v, want %v", len(args), len(tt.wantArgs))
+				return
+			}
+			
+			for i := range args {
+				if args[i] != tt.wantArgs[i] {
+					t.Errorf("buildACMEArgs()[%d] = %v, want %v", i, args[i], tt.wantArgs[i])
+				}
+			}
+		})
+	}
+}
+
+// TestACMEAvailableError tests the ACME availability error message
+func TestACMEAvailableError(t *testing.T) {
+	// We can't easily test the actual exec.LookPath call without mocking,
+	// but we can test that the function returns an error with helpful message
+	// when ACME is not found.
+	
+	// This is a simple test to ensure the function signature and basic logic
+	err := checkACMEAvailable()
+	// We can't assert much here without mocking, but we can verify the function exists
+	if err != nil && !strings.Contains(err.Error(), "ACME") {
+		t.Errorf("checkACMEAvailable() error should mention ACME, got: %v", err)
+	}
+}
+
+// TestBackwardCompatibility tests legacy flag support
+func TestBackwardCompatibility(t *testing.T) {
+	tests := []struct {
+		name        string
+		args        []string
+		wantInput   string
+		wantOutput  string
+		description string
+	}{
+		{
+			name:       "legacy -in -out flags",
+			args:       []string{"-in", "program.c65", "-out", "output.asm"},
+			wantInput:  "program.c65",
+			wantOutput: "output.asm",
+			description: "Traditional c65gm syntax",
+		},
+		{
+			name:       "legacy -in only",
+			args:       []string{"-in", "program.c65"},
+			wantInput:  "program.c65",
+			wantOutput: "program.asm",
+			description: "Legacy mode with default output",
+		},
+		{
+			name:       "mixed legacy and new",
+			args:       []string{"-in", "program.c65", "-o", "game.prg"},
+			wantInput:  "program.c65",
+			wantOutput: "game.prg",
+			description: "Mixing -in with -o",
+		},
+	}
+	
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			input, output, _, _, _, err := parseArgs(tt.args)
+			if err != nil && !strings.Contains(err.Error(), "subcommand") {
+				// parseArgs treats legacy flags as regular flags, which is correct
+				if input != tt.wantInput {
+					t.Errorf("parseArgs() input = %v, want %v", input, tt.wantInput)
+				}
+				if output != tt.wantOutput {
+					t.Errorf("parseArgs() output = %v, want %v", output, tt.wantOutput)
+				}
+			}
+		})
+	}
+}
+
+// TestEdgeCases tests various edge cases in argument parsing
+func TestEdgeCases(t *testing.T) {
+	tests := []struct {
+		name        string
+		args        []string
+		expectError bool
+		description string
+	}{
+		{
+			name:        "i flag without value",
+			args:        []string{"-i"},
+			expectError: true,
+			description: "-i flag without filename should error",
+		},
+		{
+			name:        "in flag without value",
+			args:        []string{"-in"},
+			expectError: true,
+			description: "-in flag without filename should error",
+		},
+		{
+			name:        "out flag without value",
+			args:        []string{"program.c65", "-o"},
+			expectError: true,
+			description: "-o flag without filename should error",
+		},
+		{
+			name:        "multiple input files",
+			args:        []string{"file1.c65", "file2.c65", "-o", "output.prg"},
+			expectError: false,
+			description: "First non-flag is input, second is ignored",
+		},
+		{
+			name:        "unknown flag",
+			args:        []string{"program.c65", "--unknown-flag"},
+			expectError: false,
+			description: "Unknown flags are ignored (treated as positional)",
+		},
+		{
+			name:        "help flag with other args",
+			args:        []string{"-h", "program.c65"},
+			expectError: true,
+			description: "-h flag should trigger help",
+		},
+	}
+	
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			_, _, _, _, _, err := parseArgs(tt.args)
+			
+			if tt.expectError && err == nil {
+				t.Errorf("parseArgs() expected error for %s", tt.description)
+			} else if !tt.expectError && err != nil && !strings.Contains(err.Error(), "subcommand") {
+				t.Errorf("parseArgs() unexpected error for %s: %v", tt.description, err)
+			}
+		})
+	}
+}
+
+// TestFlagEquivalence tests that -i/-in and -o/-out are equivalent
+func TestFlagEquivalence(t *testing.T) {
+	testCases := []struct {
+		args1 []string
+		args2 []string
+	}{
+		{
+			[]string{"-i", "program.c65"},
+			[]string{"-in", "program.c65"},
+		},
+		{
+			[]string{"program.c65", "-o", "output.prg"},
+			[]string{"program.c65", "-out", "output.prg"},
+		},
+		{
+			[]string{"-i", "program.c65", "-o", "output.asm"},
+			[]string{"-in", "program.c65", "-out", "output.asm"},
+		},
+	}
+	
+	for i, tc := range testCases {
+		t.Run(fmt.Sprintf("case-%d", i), func(t *testing.T) {
+			input1, output1, isBuild1, keepAsm1, noCBM1, err1 := parseArgs(tc.args1)
+			input2, output2, isBuild2, keepAsm2, noCBM2, err2 := parseArgs(tc.args2)
+			
+			// Both should succeed or both should fail
+			if (err1 == nil) != (err2 == nil) {
+				t.Errorf("parseArgs() inconsistent errors: args1=%v err=%v, args2=%v err=%v",
+					tc.args1, err1, tc.args2, err2)
+				return
+			}
+			
+			if err1 != nil {
+				return // Both failed, that's OK for this test
+			}
+			
+			// Check equivalence
+			if input1 != input2 {
+				t.Errorf("input mismatch: %q != %q", input1, input2)
+			}
+			if output1 != output2 {
+				t.Errorf("output mismatch: %q != %q", output1, output2)
+			}
+			if isBuild1 != isBuild2 {
+				t.Errorf("isBuild mismatch: %v != %v", isBuild1, isBuild2)
+			}
+			if keepAsm1 != keepAsm2 {
+				t.Errorf("keepAsm mismatch: %v != %v", keepAsm1, keepAsm2)
+			}
+			if noCBM1 != noCBM2 {
+				t.Errorf("noCBM mismatch: %v != %v", noCBM1, noCBM2)
+			}
+		})
+	}
+}
+
+// buildACMEArgs is a test helper that extracts ACME argument building logic
+func buildACMEArgs(outputFile, asmFile string, noCBM bool) []string {
+	args := []string{"-o", outputFile}
+	if !noCBM {
+		args = append(args, "-f", "cbm")
+	}
+	args = append(args, asmFile)
+	return args
+}
+
+// parseArgs is a test helper that extracts the parsing logic from main()
+func parseArgs(args []string) (inputFile, outputFile string, isBuild, keepAsm, noCBM bool, err error) {
+	if len(args) == 0 {
+		return "", "", false, false, false, fmt.Errorf("no arguments")
+	}
+	
+	// Check for help flags
+	if args[0] == "-h" || args[0] == "--help" {
+		return "", "", false, false, false, fmt.Errorf("help requested")
+	}
+	
+	// Check if first arg is a subcommand
+	if len(args) > 0 && (args[0] == "build" || args[0] == "compile" || args[0] == "help") {
+		// For testing purposes, we'll handle subcommands differently
+		// In real code, these are handled by runBuildCommand/runCompileCommand
+		return "", "", false, false, false, fmt.Errorf("subcommand not handled in test")
+	}
+	
+	// Default mode parsing (from main())
+	for i := 0; i < len(args); i++ {
+		arg := args[i]
+		
+		if arg == "-i" || arg == "-in" {
+			if i+1 < len(args) {
+				inputFile = args[i+1]
+				i++ // Skip next arg
+			} else {
+				return "", "", false, false, false, fmt.Errorf("-i flag requires a filename")
+			}
+		} else if arg == "-o" || arg == "-out" {
+			if i+1 < len(args) {
+				outputFile = args[i+1]
+				i++ // Skip next arg
+			} else {
+				return "", "", false, false, false, fmt.Errorf("-o flag requires a filename")
+			}
+		} else if arg == "--keep-asm" {
+			keepAsm = true
+		} else if arg == "--no-cbm" {
+			noCBM = true
+		} else if !strings.HasPrefix(arg, "-") && inputFile == "" {
+			// First non-flag argument is the input file
+			inputFile = arg
+		}
+	}
+	
+	if inputFile == "" {
+		return "", "", false, false, false, fmt.Errorf("no input file specified")
+	}
+	
+	// Default output based on input
+	if outputFile == "" {
+		base := strings.TrimSuffix(filepath.Base(inputFile), filepath.Ext(inputFile))
+		outputFile = base + ".prg" // Default to build mode (.prg)
+	}
+	
+	// Determine mode by output extension
+	isBuild = strings.HasSuffix(strings.ToLower(outputFile), ".prg")
+	
+	return inputFile, outputFile, isBuild, keepAsm, noCBM, nil
+}
\ No newline at end of file