From: NoOne@NoWhere.com   
      
   On 8/28/2020 1:27 AM, wolfgang kern wrote:   
   > On 28.08.2020 02:48, luserdroog wrote:   
   >> 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_lit,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....   
   >   
   >   
   > x86 Conditions:   
   > O,  NO   
   > C,  NC  aka B, NB    aka <, >=   
   > Z,  NZ  aka E, NE    aka =, <>   
   > NA, A   aka BE,NBE   aka <=,>   aka ZorC, aka NCNZ   
   > S,  NS               aka -, +   
   > PE, PO  aka P, NP   
   >   
   > L,  NL  aka NGE,GE aka  <0,>=0   
   > NG, G   aka LE,NLE aka <=0, >0   
   >   
   > only the last four and S/NS work signed   
   > __   
   > wolfgang   
   >   
      
   test   
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