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On Icarus, iCE40 (Yosys/nextpnr), and Quartus/Cyclone II tools the bit extension happens before the multiply. Operand 1 0xabcd becomes 0x0000_0000_abcd Operand 2 0x1234 becomes 0x0000_0000_1234 To fit into 16x16 multiplier, other tools are truncating the less-significant bits, resulting in a 0x0000_0000 * 0x0000_0000 multiplicaton. An unfortunate side effect is that now addition is now actually occuring, my iCE40 designs no longer meet timing. A pipeline stage may have to be added, but I am not sure how to make that play nice with the BRAMs.
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On Icarus, iCE40 (Yosys/nextpnr), and Quartus/Cyclone II tools the bit extension happens before the multiply.
Operand 1 0xabcd becomes 0x0000_0000_abcd
Operand 2 0x1234 becomes 0x0000_0000_1234
To fit into 16x16 multiplier, other tools are truncating the less-significant bits, resulting in a 0x0000_0000 * 0x0000_0000 multiplicaton.
An unfortunate side effect is that now addition is now actually occuring, my iCE40 designs no longer meet timing. A pipeline stage may have to be added, but I am not sure how to make that play nice with the BRAMs.