Changing the processor flags

As seen in the previous chapter, the SNES supports 9 processor flags. There are ways to affect these processor flags, which are explained in this chapter.

SEP

Opcode	Full name	Explanation
SEP	Set processor flags	Sets specified processor flags to 1

SEP sets the selected processor flag bits to 1.

SEP is used as following:

;nvmxdizc = 0000 0000
SEP #$80 ;= 1000 0000
;Nvmxdizc = 1000 0000

The uppercased letters are the activated processor flags. This code sets the negative flag.

REP

Opcode	Full name	Explanation
REP	Reset processor flags	Sets specified processor flags to 0

REP resets the selected processor flag bits to 0.

REP is used as following:

;NvmxDizc = 1000 1000
REP #$08 ;= 0000 1000
;Nvmxdizc = 1000 0000

In the beginning, the decimal mode was enabled, and the negative flag was set, but after REP #$08, the decimal mode flag got disabled and the negative flag is still set.

XCE and the emulation mode

Because the emulation mode bit is "hidden" above the carry flag, there's a dedicated opcode which exchanges the carry flag with the emulation mode flag.

Opcode	Full name	Explanation
XCE	Exchange carry and emulation	Swaps the values of carry flag and emulation mode flag

You can access the emulation mode by using SEC then XCE, and leave it using CLC then XCE. By default, SNES starts up in emulation mode. This is why you often see the opcodes CLC and XCE among the very first opcodes in disassemblies.

Other opcodes

There are other opcodes which immediately affect a single processor flag.

Opcode	Full name	Explanation
SEI	Set interrupt disable flag	Sets the interrupt disable flag, setting it to 1, thus disabling IRQ
CLI	Clear interrupt disable flag	Clears the interrupt disable flag, setting it to 0, thus enabling IRQ
SED	Set decimal flag	Sets the decimal mode flag, setting it to 1, changing the SNES into decimal mode
CLD	Clear decimal flag	Clears the decimal flag, setting it to 0, changing the SNES into hexadecimal mode
SEC	Set carry flag	Sets the carry flag, setting itt o 1
CLC	Clear carry flag	Clears the carry flag, setting it to 0
CLV	Clear overflow flag	Clears the overflow flag, setting it to 0