safe VSP for mayhem

safe VSP channel explained, for mayhem in monsterland release:

disk original code:

$213d- starts irq stabilization (TSX TXS method)

.C:2196  20 82 B1    JSR $B182		;delay
.C:2199  A0 1B       LDY #$1B
.C:219b  A9 12       LDA #$12
.C:219d  8D 11 D0    STA $D011		;rasterline is $50 and cycle is 50 here
.C:21a0  8C 11 D0    STY $D011
.C:21a3  8E 12 D0    STX $D012
.C:21a6  A2 FC       LDX #$FC
.C:21a8  9A          TXS
.C:21a9  A9 1F       LDA #$1F
.C:21ab  8D 18 D0    STA $D018
.C:21ae  A9 00       LDA #$00
.C:21b0  A2 06       LDX #$06
.C:21b2  A0 00       LDY #$00
.C:21b4  40          RTI

safe VSP mod :

.C:2181  A2 1B       LDX #$1B		;this is inserted inside irq routine, value is $1B EOR (safe channel #)
.C:2183  A5 10       LDA $10
.C:2185  09 10       ORA #$10
.C:2187  8D 16 D0    STA $D016
.C:218a  AD 12 D0    LDA $D012
.C:218d  C9 30       CMP #$30
.C:218f  F0 00       BEQ $2191
.C:2191  A5 0F       LDA $0F
.C:2193  4A          LSR A
.C:2194  90 00       BCC $2196
.C:2196  20 82 B1    JSR $B182		;delay
.C:2199  A0 1B       LDY #$1B
.C:219b  A9 12       LDA #$12
.C:219d  8D 11 D0    STA $D011		;like above, rasterline should be $50 and cycle should be 50 here
.C:21a0  8E 11 D0    STX $D011		;this is inserted for safe VSP
.C:21a3  8C 11 D0    STY $D011

so, plot is, rewrite or modify the stable irq routine with adding LDX#$value before (to be stored into $D011) and
between DEC$D011 ($12 here or $1A) and INC$D011 ($1B), store X value into $D011, without breaking the cycle-exact timing.
result is : safe VSP works same as LFT's VSP-LAB release (first introduced in his SAFE VSP demo).