Subtraction is identical to ADD, but the second operand is negated (like A-B=A+(-B)).
Short forms :
The second operand (snd) is subtracted from the first operand
(si4, either a register or a sign-extended 4-bit value)
and the result is written into the second operand (snd).
Long immediate version : The source register (snd) is subtracted from immediate field (Imm16, sign-extended with YASEP32) and the result is written to the destination register (si4).
; D4 = 123h ; R1 = 1h SUB R1 D4 ; D4= D4+(-R1) = 123h+FFFFFFFEh = 122h SUB 4 R1 D4 ; D4= 4h+(-R1) = 4h+FFFFFFFEh = 3h
Note that subtraction by an immediate value is possible by ADD-ing a negative immediate value.
; R1 = 5678h ADD -4 R1 D4 ; D4= 5678h+(-4) = 5674h
NEGation of a register is possible with an alias that subtracts this register from immediate 0.
The result is truncated to the 16 (YASEP16) or 32 (YASEP32) lower bits.
The borrow bit is the same as the carry bit, accessed with the CARRY condition.
Due to architectural optimisations, the borrow bit is negated :
it is set when the subtraction did not borrow.
; R1 = 4h SUB 3 R1 ; R1 = 3 - 4 = -1 ==> borrow=0 SUB 4 R1 ; R1 = 4 - 4 = 0 ==> borrow=1 SUB 5 R1 ; R1 = 5 - 4 = 1 ==> borrow=1