diff --git a/ka/usr/cmd-exec.k b/ka/usr/cmd-exec.k
index 6cac8d9..ee0afdd 100644
--- a/ka/usr/cmd-exec.k
+++ b/ka/usr/cmd-exec.k
@@ -11,21 +11,25 @@
     blte zero, eax, .do_exec
 
     ; nope, append ".com" and try again
+    mov esi, argv0
     sub ecx, argbuf.size, 5
-    scasb ax0, zero ; find null-term
-    mov b[ax0+0], '.' ; ".com"
-    mov b[ax0+1], 'c'
-    mov b[ax0+2], 'o'
-    mov b[ax0+3], 'm'
-    nul b[ax0+4]
+    scasb esi, zero ; find null-term
+    mov b[esi], '.' ; ".com"
+    mov b[esi+1], 'c'
+    mov b[esi+2], 'o'
+    mov b[esi+3], 'm'
+    nul b[esi+4]
 
     ; try again
     mov eax, Sys.OpenFile
     mov ax0, argv0
     trap 0
 
-    ; still no good?
-    bltz eax, .exec_not_found
+    ; we good?
+    blte 0, eax, .do_exec
+
+    nul b[esi]    ; erase ".com"
+    jmp .exec_not_found
 
 .do_exec:
     ; load file into memory
diff --git a/vm/ob/command.k b/vm/ob/command.k
index a5683a0..29f5544 100644
--- a/vm/ob/command.k
+++ b/vm/ob/command.k
@@ -1380,21 +1380,25 @@ main:
     blte zero, eax, .do_exec
 
     ; nope, append ".com" and try again
+    mov esi, argv0
     sub ecx, argbuf.size, 5
-    scasb ax0, zero ; find null-term
-    mov b[ax0+0], '.' ; ".com"
-    mov b[ax0+1], 'c'
-    mov b[ax0+2], 'o'
-    mov b[ax0+3], 'm'
-    nul b[ax0+4]
+    scasb esi, zero ; find null-term
+    mov b[esi], '.' ; ".com"
+    mov b[esi+1], 'c'
+    mov b[esi+2], 'o'
+    mov b[esi+3], 'm'
+    nul b[esi+4]
 
     ; try again
     mov eax, Sys.OpenFile
     mov ax0, argv0
     trap 0
 
-    ; still no good?
-    bltz eax, .exec_not_found
+    ; we good?
+    blte 0, eax, .do_exec
+
+    nul b[esi] ; erase ".com"
+    jmp .exec_not_found
 
 .do_exec:
     ; load file into memory