use firmware calls for RND

README.md

@@ -1,16 +1,24 @@
-# mazecpc 1.1
-Maze CPC is a maze generator in 14 bytes for Amstrad CPC series 8-bit computers.
-
-![screen shot](https://raw.githubusercontent.com/ssg/mazecpc/master/screenshot.png)
-
-# history
-There is a book called "[10 PRINT CHR$ (205.5 + RND (1)); : GOTO 10](http://10print.org/)". Yes that's the  title. It's about a small code piece in BASIC language that generates a maze. An interesting read for programmers. It delves into attempts to create it in assembly as small as possible, which [seems to have become a trend](http://csdb.dk/release/?id=113300) in Commodore 64 demoscene.
-
-Recently I saw [a blog post from Trixter](http://trixter.oldskool.org/2012/12/17/maze-generation-in-thirteen-bytes/) who tried to port it to PC. It eventually got down to 10 bytes.
-
-Tonight I wanted to give it a shot on Amstrad CPC and here it is, <s>11 bytes</s>14-bytes maze generator, created in couple of hours.
-
-# technical notes
-I found it very hard to produce a series of "random bits" on a Z80 CPU. There is R register which increases every cycle but naturally deterministic in loops. To keep the code short, however, I relied on it and interrupts to introduce a slight entropy. I'm sure there is room for improvement. Update Dec 29th, 2018: I made the random generator much better by sacrificing 3 more bytes. It's a 14-byte maze generator now.
-
-SSG / arteffect, March 24th 2015
+# mazecpc 1.2
+
+Maze CPC is a maze generator in 12 bytes for Amstrad CPC664/6128 8-bit computers.
+
+![screen shot](https://raw.githubusercontent.com/ssg/mazecpc/master/screenshot.png)
+
+# history
+
+There is a book called "[10 PRINT CHR$ (205.5 + RND (1)); : GOTO 10](http://10print.org/)". Yes that's the title. It's about a small code piece in BASIC language that generates a maze. An interesting read for programmers. It delves into attempts to create it in assembly as small as possible, which [seems to have become a trend](http://csdb.dk/release/?id=113300) in Commodore 64 demoscene.
+
+I saw [a blog post from Trixter](http://trixter.oldskool.org/2012/12/17/maze-generation-in-thirteen-bytes/) who tried to port it to PC. It eventually got down to 10 bytes.
+
+Tonight I wanted to give it a shot on Amstrad CPC and here it is, <s>11</s> <s>14</s> 12-bytes maze generator, created in a couple of hours.
+
+# technical notes
+
+I found it very hard to produce a series of "random bits" on a Z80 CPU. There is R register which increases every cycle but naturally deterministic in loops. To keep the code short, however, I relied on it and interrupts to introduce a slight entropy. I'm sure there is room for improvement.
+
+_Update Dec 29th, 2018: I made the random generator much better by sacrificing 3 more bytes. It's a 14-byte maze generator now._
+
+_Update May 2nd, 2023: I resorted to using firmware call for RND to get the random number,
+saving another 2 bytes. But, the code doesn't run on CPC464 now due to missing firmware call. The 14-byte CPC464 version still exists in `maze464.asm`_
+
+SSG / arteffect, March 24th 2015

maze.asm

@@ -1,18 +1,27 @@
-org         &4000
-write direct "maze.bin"
-
-CHAR        equ &cc
-TXT_WR_CHAR equ &bb5d
-
-run $
-
-.loop:  
-  ld    a, r
-  rrca
-  rrca
-  rrca
-  and   1
-  or    CHAR
-  call  TXT_WR_CHAR  
-  jr    loop  
-
+; This is the 12-byte CPC664/CPC6128 version.
+
+org         &4000
+write "maze.bin"
+write direct "maze.bin"
+
+CHAR        equ &CC
+TXT_WR_CHAR equ &BB5D
+REAL_RND    equ &BD7F ; CPC464 doesn't have this endpoint.
+
+run $
+loop:  
+	; REAL_RND call normally works on the memory at HL register
+	; we ignore that and use the destroyed A value because somehow
+	; it also varies quite randomly, saving us 2 bytes in the end.
+	; This trick causes RND to output junk pixels to the scroll 
+	; position due to HL value destroyed by TXT_WR_CHAR, but it 
+	; gets painted over anyway.
+	call  REAL_RND
+	and   1
+	or    CHAR
+	call  TXT_WR_CHAR  
+	jr    loop
+
+result:
+
+close

maze.bin

@@ -1 +0,0 @@
-�_�
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winape.asm

@@ -1,2 +1,2 @@
-; this is to avoid using winape's own editor
+; this is to avoid using winape's own editor
 read "maze.asm"