FLEX is the name of the Operating System for the Motorola 6800 and 6809. IDA can disassemble OS9 object and FLEX STX files and is probably the most powerful disassembler for that platform.
Assembler code
TEXT:54A5
TEXT:54A5 * =============== S U B R O U T I N E =======================================
TEXT:54A5
TEXT:54A5
TEXT:54A5 sub_54A5 * CODE XREF: sub_4F06+78↑P
TEXT:54A5 * sub_5048+1CB↑P ...
TEXT:54A5 pshs dpr,y,u
TEXT:54A7 ldy 7,s
TEXT:54AA ldx ,y
TEXT:54AC ldd 9,s
TEXT:54AE aslb
TEXT:54AF leay b,y
TEXT:54B1 ldd $B,s
TEXT:54B3 pshs a,b,x
TEXT:54B5 tfr b,a
TEXT:54B7 ldu ,y
TEXT:54B9 beq loc_54BD
TEXT:54BB jsr ,u
TEXT:54BD
TEXT:54BD loc_54BD * CODE XREF: sub_54A5+14↑j
TEXT:54BD tfr a,b
TEXT:54BF clra
TEXT:54C0 leas 4,s
TEXT:54C2 puls pcr,u,y,dpr
TEXT:54C2 * End of function sub_54A5
TEXT:54C2
TEXT:54C4
TEXT:54C4 * =============== S U B R O U T I N E =======================================
TEXT:54C4
TEXT:54C4
TEXT:54C4 sub_54C4 * CODE XREF: sub_4C67+63↑P
TEXT:54C4 * sub_4C67+87↑P ...
TEXT:54C4 pshs dpr,y,u
TEXT:54C6 lda $C,s
TEXT:54C8 ldx 7,s
