Description
The contents of register rA are subtracted from the contents of register rB and the result is placed into register rD. Bit 3 of the instruction (labeled as K in the figure) is set to one for the mnemonic rsubk. Bit 4 of the instruction (labeled as C in the figure) is set to one for the mnemonic rsubc. Both bits are set to one for the mnemonic rsubkc.
When an rsub instruction has bit 3 set (rsubk, rsubkc), the carry flag will Keep its previous value regardless of the outcome of the execution of the instruction. If bit 3 is cleared (rsub, rsubc), then the carry flag will be affected by the execution of the instruction.
When bit 4 of the instruction is set to one (rsubc, rsubkc), the content of the carry flag (MSR[C]) affects the execution of the instruction. When bit 4 is cleared (rsub, rsubk), the content of the carry flag does not affect the execution of the instruction (providing a normal subtraction).
Pseudocode
if C = 0 then
(rD) ← (rB) + (rA) + 1
else
(rD) ← (rB) + (rA) + MSR[C]
if K = 0 then
MSR[C] ← CarryOutRegisters Altered
- rD
- MSR[C]
Latency
- 1 cycle
Note
- In subtractions, Carry = (Borrow). When the Carry is set by a subtraction, it means that there is no Borrow, and when the Carry is cleared, it means that there is a Borrow.