NUT CPU BCD arithmetics in SETDEC mode (aka Goose hunt)

04022024, 10:20 AM
Post: #1




NUT CPU BCD arithmetics in SETDEC mode (aka Goose hunt)
Hi all,
As have been noted in previous threads, the main reason for failing to fly the goose backwards in all emulators and simulators so far seems to be a shortage in the emulation of BCD arithmetics in the NUT CPU. If the syntethic FIX 10 instruction is executed, this results in a hexadecimal value being decremented in decimal mode causing the emulators and simulators to differ from the real HP41 causing the goose to be missed! I have tried to trace and understand the underlying logic (and as Christoph mentioned  this is of course the gate logic in the NUT CPU) when doing BCD arithmetics in decimal mode. I have added some results taken from a real HP41. All arithmetic is done on the S&X field (12 bits) of register A. I have added or subtracted 1, 5 and F from all possible values (000FFF) and traced the result and checked if carry was set or not. All logs have the same layout: Code: A[S&X] (original value) The case with adding 1 was easy to understand. First the value is translated to decimal digit by digit (A=10, .. F=15) Code: int d = 0; The result is incremented by 1, and if the result overflows the field, then carry is set. Some examples: Code: 000 (000 + 00 + 0 = 000) > 000 + 1 = 001 Subtracting with 1 can not be done with a similar approach, eg: Code: 00F > 008 And adding/subtracting a different number makes it even worse. E.g. adding 0xF in decimal mode results sometimes in hexadecimal values: Code: 000 + 00F > 015 It is not too hard to come up with some code that will handle these cases, but I have trouble finding a common solution that will handle all cases and all fields. Maybe someone with knowledge about BCD algorithms and/or gate logic can see some patterns here ... ? Feel free to suggest some other tests that I could provide the results from. (Will add some more traces in next post.) Cheers, Thomas [35/45/55/65/67/97/80 21/25/29C 31E/32E/33EC/34C/38E 41CCVCX 71B 10C/11C/12C/15CCE/16C 32SSII/42S 28CS 48GX/49G/50G 35S 41X] 

04022024, 10:22 AM
Post: #2




RE: NUT CPU BCD arithmetics in SETDEC mode (aka Goose hunt)
Added some more traces ...
Cheers, Thomas [35/45/55/65/67/97/80 21/25/29C 31E/32E/33EC/34C/38E 41CCVCX 71B 10C/11C/12C/15CCE/16C 32SSII/42S 28CS 48GX/49G/50G 35S 41X] 

04032024, 09:54 AM
Post: #3




RE: NUT CPU BCD arithmetics in SETDEC mode (aka Goose hunt)
TNX Thomas,
verified the V41Adder and fixed the Subtraction to base 10 with hexadecimal numbers. With your delivered tables it would be a shame to verify only single points, so I created my own tables to compare them with your references. Christoph 

04032024, 11:34 AM
Post: #4




RE: NUT CPU BCD arithmetics in SETDEC mode (aka Goose hunt)
(04032024 09:54 AM)Christoph Giesselink Wrote: With your delivered tables it would be a shame to verify only single points, so I created my own tables to compare them with your references. Hi Christoph, Please let me know if you need to test some other corner cases! I tried to implement a BCDadder, and addition worked fine (gives 100% same result as my traces), but I can't manage to crack the subtraction ... In general, a BCD subtraction is done by complementing (either by 9 or 10) one value and feed it to an adder (A  B = A + ~B). But, it doesn't work, there seems to be some special handling when e.g. hex:A is part of the value, so it is not as straight forward as addition. Cheers, Thomas [35/45/55/65/67/97/80 21/25/29C 31E/32E/33EC/34C/38E 41CCVCX 71B 10C/11C/12C/15CCE/16C 32SSII/42S 28CS 48GX/49G/50G 35S 41X] 

04032024, 05:26 PM
Post: #5




RE: NUT CPU BCD arithmetics in SETDEC mode (aka Goose hunt)
(04032024 11:34 AM)ThomasF Wrote: I tried to implement a BCDadder, and addition worked fine (gives 100% same result as my traces), but I can't manage to crack the subtraction ... I haven't tried it to make it by boolean operations. It's just a fix of the existing code using BASECY as BASE overflow indicator (opcode examples incomplete). Code: ... Code: /****************************/ As old Z80 coder I remembered the Decimal Adjust (DAA) instruction in the Z80 CPU. So I had I look into a Zilog User Manual: Decimal Adjust Accumulator Flag The Decimal Adjust Accumulator (DAA) instruction uses this flag to distinguish between ADD and SUBTRACT instructions. For all ADD instructions, N sets to 0. For all SUBTRACT instructions, N sets to 1. Half Carry Flag The Half Carry Flag (H) is set (1) or cleared (0) depending on the Carry and Borrow status between bits 3 and 4 of an 8bit arithmetic operation. This flag is used by the Decimal Adjust Accumulator (DAA) instruction to correct the result of a packed BCD add or subtract operation. The H Flag is set (1) or cleared (0) as shown in Table 23. So for coding DAA it needs 3 flags, NFlag, HFlag und CFlag, quite complex. No further investigation. 

04042024, 07:31 AM
Post: #6




RE: NUT CPU BCD arithmetics in SETDEC mode (aka Goose hunt)
Thanks Christoph!
It was arithmetics like these that fooled me (subtraction with F): Code: A10 > 40B CARRY Anyhow, using the algorithm in Subtractor that you provided works correct, so that could be used to implement BCD subtraction. Using the following code, I could verify with all examples in previous traces! Code: byte Subtractor(byte nib1, byte nib2) Cheers, Thomas [35/45/55/65/67/97/80 21/25/29C 31E/32E/33EC/34C/38E 41CCVCX 71B 10C/11C/12C/15CCE/16C 32SSII/42S 28CS 48GX/49G/50G 35S 41X] 

04242024, 05:37 PM
Post: #7




RE: NUT CPU BCD arithmetics in SETDEC mode (aka Goose hunt)
(04042024 07:31 AM)ThomasF Wrote: Using the following code, I could verify with all examples in previous traces! On my request Thomas made an additional test with the ?A<C and ?A<B opcodes. Like on other CPU's the compare function base on a subtraction without saving the result. So ?A<C base on the subtraction of AC and has the same side effects on the Carry Flag like A=AC. So the less function for the NUT CPU could be implemented as: Code: void bcd_less(char *p1, char *p2, int n) The complete source code of V41 R9L with a reference implementation is now available. Christoph 

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