From: luser.droog@nospicedham.gmail.com   
      
   On Thursday, August 27, 2020 at 3:12:28 PM UTC-5, luserdroog wrote:   
   > Hi everyone,   
   >   
   > I'm working on primitives for my newly working forth interpreter and I'm   
   > having difficulty with comparisons 0= 0> and 0> . These each take a   
   > number from the stack and push 0 or all-bits-1.   
   >   
   > My code for 0= and 0> appear to be producing results, but I get the wrong   
   > result for 0< even though it seems to follow appropriate parallel logic   
   > as 0>. Any ideas? Does this suggest a bug in assembly or in my emulator?   
   >   
   > CODE(0=,     zeroeq,    POP(AX),MOVCXI(1,0), CMP(,R,AX,CX), SBB(,R,AX,AX),   
   PUSH(AX))   
   > CODE(0>,     zerogt,    POP(AX),MOVCXI(0,0), CMP(,R,AX,CX), JL   
   3,MOVCXI(0xff,0xff), PUSH(CX))   
   > CODE(0<,     zerolt,    POP(AX),MOVCXI(0,0), CMP(,R,AX,CX), JG   
   3,MOVCXI(0xff,0xff), PUSH(CX))   
   >   
   > testing:   
   > WORD(run,    run,       c_lit,0, c_zeroeq, c_dot,   
   >                         c_lit,12, c_zerogt, c_dot,   
   >                         c_lit,minus(50), c_zerolt, c_dot,   
   >                         c_lit,79,c_emit, c_lit,75,c_emit, c_li   
   ,5,c_double,c_emit, c_bye)   
   >   
   > output:   
   > $ ./a8086   
   > 110OK   
      
   I changed JL to JLE and JG to JGE. Still not right.   
   Here's a trace through zerogt.   
      
   ax:01f6 cx:0001 dx:0030 bx:01f8 sp:0ffe bp:2000 si:05ac di:0000 ip:01f8   
   fl:0004 NC NO NS NZ   
   58(130) popax:   
   ax:000c cx:0001 dx:0030 bx:01f8 sp:1000 bp:2000 si:05ac di:0000 ip:01f9   
   fl:0004 NC NO NS NZ   
   b9(271) movcxi: 00(000) 00(000)   
   ax:000c cx:0000 dx:0030 bx:01f8 sp:1000 bp:2000 si:05ac di:0000 ip:01fc   
   fl:0004 NC NO NS NZ   
   3b(073) cmpwt: c8(310) x:0 y:12 ->fffffff4   
   ax:000c cx:0000 dx:0030 bx:01f8 sp:1000 bp:2000 si:05ac di:0000 ip:01fe   
   fl:0891 CA OV SN NZ   
   7e(176) jle: 03(003) <0>   
   ax:000c cx:0000 dx:0030 bx:01f8 sp:1000 bp:2000 si:05ac di:0000 ip:0200   
   fl:0891 CA OV SN NZ   
   b9(271) movcxi: ff(377) ff(377)   
   ax:000c cx:ffff dx:0030 bx:01f8 sp:1000 bp:2000 si:05ac di:0000 ip:0203   
   fl:0891 CA OV SN NZ   
   51(121) pushcx:   
   ax:000c cx:ffff dx:0030 bx:01f8 sp:0ffe bp:2000 si:05ac di:0000 ip:0204   
   fl:0891 CA OV SN NZ   
      
      
   It should be comparing 0 and 12, so it should take 0-12 and set   
   zf=0 sf=1 and of=1. Then JLE should jump if (sf^of)|zf which is 0.   
   And it seems to be doing all that. Maybe I'm just using CMP wrong?   
      
   The description from the 8086 family manual:   
      
   CMP destination,source   
   CMP (Compare) subtracts the source from the destination, which may be   
   bytes or words, but does not return the result. The operands are   
   unchanged, but the flags are updated and can be tested by a subsequent   
   conditional jump instruction. CMP updates AF, CF, OF, PF, SF and ZF.   
   The comparison reflected in the flags is that of the destination to   
   the source. IF a CMP instruction is followed by a JG (jump if greater)   
   instruction, for example, the jump is taken if the destination operand   
   is greater than the source operand.   
      
   So, in "CMP CX=0,AX=12" CX is the destination. So 0-12. Hmmm.   
   I still don't get it. But all the pieces seem to be here....   
      
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