diff --git a/doc/zasm.md b/doc/zasm.md
index bef3822..9eff004 100644
--- a/doc/zasm.md
+++ b/doc/zasm.md
@@ -5,28 +5,22 @@ from within the shell. What you need is:
 
 * A mounted filesystem with `zasm` on it.
 * A block device to read from (can be a file from mounted CFS)
-* A block device to write to (can theoretically be a file, but technical
-  limitations temporary prevents us that. We'll use a mmap for now).
+* A block device to write to (can also be a file).
 
 The emulated shell is already set up with all you need. If you want to run that
 on a real machine, you'll have to make sure to provide these requirements.
 
 The emulated shell has a `hello.asm` file in its mounted filesystem that is
 ready to compile. It has two file handles 0 and 1, mapped to blk IDs 1 and 2.
-We only use file handle 0 (blk ID 1) and then tell zasm to output to mmap which
-is configured to start at `0xe00`
+We will open our source file in handle 0 and our dest file in handle 1. Then,
+with the power of the `pgm` module, we'll autoload our newly compiled file and
+execute it!
 
     Collapse OS
-    > fopn 0 hello.asm      ; open file in handle 0
-    > zasm 1 3              ; assemble opened file and spit result in mmap
-    > bsel 3                ; select mmap
-    > mptr e000             ; set memptr to mmap's beginning
-    > peek 5
-    210890CD3C              ; looking good
-    > mptr 4200             ; hello.asm is configured to run from 0x4200
-    > load ff               ; load compiled code from mmap
-    > peek 5
-    210890CD3C              ; looking good
-    > call 00 0000
+    > fnew 1 dest           ; create destination file
+    > fopn 0 hello.asm      ; open source file in handle 0
+    > fopn 1 dest           ; open dest binary in handle 1
+    > zasm 1 3              ; assemble source file into binary file
+    > dest                  ; call newly compiled file
     Assembled from the shell
     >                       ; Awesome!
diff --git a/tools/emul/shell/shell.c b/tools/emul/shell/shell.c
index e6941c1..5fac826 100644
--- a/tools/emul/shell/shell.c
+++ b/tools/emul/shell/shell.c
@@ -32,9 +32,11 @@
 #define FS_DATA_PORT 0x01
 // Controls what address (24bit) the data port returns. To select an address,
 // this port has to be written to 3 times, starting with the MSB.
-// Reading this port returns an out-of-bounds indicator. 0 means addr is within
-// bounds, non zero means either that we're in the middle of an addr-setting
-// operation or that the address is not within bounds.
+// Reading this port returns an out-of-bounds indicator. Meaning:
+// 0 means addr is within bounds
+// 1 means that we're equal to fsdev size (error for reading, ok for writing)
+// 2 means more than fsdev size (always invalid)
+// 3 means incomplete addr setting
 #define FS_ADDR_PORT 0x02
 
 static Z80Context cpu;
@@ -74,8 +76,11 @@ static uint8_t io_read(int unused, uint16_t addr)
         }
     } else if (addr == FS_ADDR_PORT) {
         if (fsdev_addr_lvl != 0) {
-            return fsdev_addr_lvl;
-        } else if (fsdev_ptr >= fsdev_size) {
+            return 3;
+        } else if (fsdev_ptr > fsdev_size) {
+            fprintf(stderr, "Out of bounds FSDEV addr request at %d / %d\n", fsdev_ptr, fsdev_size);
+            return 2;
+        } else if (fsdev_ptr == fsdev_size) {
             return 1;
         } else {
             return 0;
@@ -101,11 +106,17 @@ static void io_write(int unused, uint16_t addr, uint8_t val)
             return;
         }
         if (fsdev_ptr < fsdev_size) {
+#ifdef DEBUG
+            fprintf(stderr, "Writing to FSDEV (%d)\n", fsdev_ptr);
+#endif
             fsdev[fsdev_ptr] = val;
         } else if ((fsdev_ptr == fsdev_size) && (fsdev_ptr < MAX_FSDEV_SIZE)) {
             // We're at the end of fsdev, grow it
             fsdev[fsdev_ptr] = val;
             fsdev_size++;
+#ifdef DEBUG
+            fprintf(stderr, "Growing FSDEV (%d)\n", fsdev_ptr);
+#endif
         } else {
             fprintf(stderr, "Out of bounds FSDEV write at %d\n", fsdev_ptr);
         }
diff --git a/tools/emul/shell/shell_.asm b/tools/emul/shell/shell_.asm
index 77fd0f5..c18d3ec 100644
--- a/tools/emul/shell/shell_.asm
+++ b/tools/emul/shell/shell_.asm
@@ -125,8 +125,8 @@ fsdevPutC:
 	ld	a, l
 	out	(FS_ADDR_PORT), a
 	in	a, (FS_ADDR_PORT)
-	or	a
-	jr	nz, .error
+	cp	2		; only A > 1 means error
+	jr	nc, .error	; A >= 2
 	pop	af
 	out	(FS_DATA_PORT), a
 	cp	a		; ensure Z