The -F number is the seed used by the pMARS pseudorandom number generator. An -F number exploit uses a known seed to calculate the opponent's starting positions and launch a targeted attack. Björn Günzel suggested the exploit in May 1996, Nathan Green coded a demonstration shortly after and Dave Hillis described the technique in Core Warrior issue 79.

Macro -F Exploit

A simple example is to emulate the random number generator in macros to create a table of starting positions. p-space is used to maintain a pointer into the table and the opponent is hit swiftly by a simple bomber.

        fnumber equ 2345
        count  equ 123

start   ldp.a  #count,   table
        mov.x  }table,   *table
        stp.ab table,    #count
        mod.a  #tlen+1,  table
        mov    *table,   topptr
        mov    topptr,   endptr

        ptroff equ 6252
        topptr equ (start+ptroff+2)
        endptr equ (start+ptroff+95)

        spl    #0,       0
bomber  mov    >topptr,  <endptr
        mov    <endptr,  >topptr
        mov    >start+2, >topptr
        djn.f  bomber,   <endptr

; temp = 16807 * (temp % 127773) - 2836 * (temp / 127773)
; if (temp < 0) temp += 2147483647

; macro for CORESIZE 8000, MINDISTANCE 100

posa equ (t=((t=16807*(t%127773)-2836*(t/127773))+(t<0)*2147483647))%7801+100-ptroff
posb equ (u=((u=16807*(u%127773)-2836*(u/127773))+(u<0)*2147483647))%7801+100-ptroff

        tlen   equ (MAXLENGTH-11)

        for    tlen-1+0*t=u=fnumber-100
        for    0*posb

table   spl    #0,       <posb
        for    MAXLENGTH-CURLINE
        mov    >posa,    <posb