8.13. DEMOS
we can read about one of the most simple possible random maze generators.
It just prints a slash or backslash characters randomly and endlessly, resulting in something like this:
There are a few known implementations for 16-bit x86.
Trixter’s 42 byte version
The listing was taken from his website^47 , but the comments are mine.
00000000: B001 mov al,1 ; set 40x25 video mode
00000002: CD10 int 010
00000004: 30FF xor bh,bh ; set video page for int 10h call
00000006: B9D007 mov cx,007D0 ; 2000 characters to output
00000009: 31C0 xor ax,ax
0000000B: 9C pushf ; push flags
; get random value from timer chip
0000000C: FA cli ; disable interrupts
0000000D: E643 out 043,al ; write 0 to port 43h
; read 16-bit value from port 40h
0000000F: E440 in al,040
00000011: 88C4 mov ah,al
00000013: E440 in al,040
00000015: 9D popf ; enable interrupts by restoring IF flag
00000016: 86C4 xchg ah,al
; here we have 16-bit pseudorandom value
00000018: D1E8 shr ax,1
0000001A: D1E8 shr ax,1
; CF currently have second bit from the value
0000001C: B05C mov al,05C ;'\'
; if CF=1, skip the next instruction
0000001E: 7202 jc 000000022
; if CF=0, reload AL register with another character
00000020: B02F mov al,02F ;'/'
; output character
00000022: B40E mov ah,00E
00000024: CD10 int 010
00000026: E2E1 loop 000000009 ; loop 2000 times
00000028: CD20 int 020 ; exit to DOS
The pseudo-random value here is in fact the time that has passed from the system’s boot, taken from the
8253 time chip, the value increases by one 18.2 times per second.
By writing zero to port43h, we send the command “select counter 0”, ”counter latch”, ”binary counter”
(not aBCDvalue).