Restrict input length for ZX Spectrum

* Update the input routine to cap input at a sane size, to avoid
  scrolling on input.
* Reworded "invalid command" message to fit on the spectrum screen.

Author: skx

bios.z80

@@ -20,6 +20,11 @@
 ; platform-specific code we need.
 ;
 
+IF SPECTRUM
+  MAX_INPUT_LENGTH: EQU 30
+ELSE
+  MAX_INPUT_LENGTH: EQU 78
+ENDIF
 
 ;
 ; Initialization routine for the BIOS, if needed
@@ -236,15 +241,15 @@ IF SPECTRUM
 
         xor a
         ld (hl),a  ; size is now zero
-        inc hl ; point to the character
+        inc hl     ; point to the start of the character buffer
 
 read_input_again:
         ; read a char
         call bios_await_keypress
 
         ; return?  Then we're done
         cp 13
-        jr z, read_input_over
+        jp z, read_input_over
 
         ; delete?  remove the last character
         cp 12
@@ -254,6 +259,25 @@ read_input_again:
         cp 7
         jr z, reset_line
 
+        ; too many characters?
+        push de
+        push af
+          ld a,(de)                  ; get the count of characters we've entered
+          cp MAX_INPUT_LENGTH        ; ignore input if too long
+          jr nc,too_many_characters
+          jr z, too_many_characters
+        pop af
+        pop de
+        jr continue_input
+
+too_many_characters:
+        ; too many characters, cleanup stack and ignore the character
+        pop af
+        pop de
+        jr read_input_again
+
+continue_input:
+
         ; display the new character
         push de
         ld e,a
@@ -344,7 +368,7 @@ loopy:
         pop bc
         pop de
         pop hl
-        jr read_input_again
+        jp read_input_again
 
 reset_line:
         ; clear the screen, and restart the input process

game.z80

@@ -2729,8 +2729,8 @@ NEWLINE:
 TAB:
         db "     $"
 invalid_msg:
-        db 0x0a, 0x0d, "I did not understand your command.", 0x0a, 0x0d
-        db "For a list of several of our recognized words type 'HELP'.", 0x0a, 0x0d, "$"
+        db 0x0a, 0x0d, "I did not understand your input.", 0x0a, 0x0d, 0x0a, 0x0d
+        db "Enter 'HELP' to see some of our commands.", 0x0a, 0x0d, "$"
 BAD_LANGUAGE_MSG:
         db 0x0a, 0x0d, "What language!", 0x0a, 0x0d, "$"
 NO_PHONE_HERE:
@@ -3607,8 +3607,8 @@ ITEM_COUNT: EQU ( item_last - items ) / ITEM_ENTRY_LENGTH
 ;    http://www.gaby.de/cpm/manuals/archive/cpm22htm/ch5.htm#Function_10
 ;
 INPUT_BUFFER:
-        db 100  ; size of our input-buffer
-        db 0x00 ; Count of characters returned.
+        db MAX_INPUT_LENGTH  ; size of our input-buffer
+        db 0x00              ; Count of characters returned.