So here's the code. DeclareMPCall is a macro to put a pointer to NKDebug in the "MPCall" (syscall) table at position 200. By default only about 130 positions in that table are used. I will soon write an app to extract the entire kernel output from an unmodified kernel.
;_______________________________________________________________________
; My additions to the NanoKernel, to go at the end of the code image
;_______________________________________________________________________
if NKDebugShim && !Vanilla
DeclareMPCall 200, NKDebug
NKDebug
; Lifted from NKxprintf:
; Put the physical address of the r3 arg in r8
rlwinm. r9, r11, 0, DRbit, DRbit ; IntSyscall sets this
mr r8, r3
beq- @already_physical
li r9, 0
bl V2P ; takes page EA in r8, r9=0, returns page PA in r17
beq- @fail
rlwimi r8, r17, 0, 0, 19
@already_physical
; Copy the command into the KDP buffer reserved for this purpose:
; r8 = src
; r29 = dest
; r30 = ctr
; r31 = val
mfsprg r1, 0
lwz r1, EWA.PA_KDP(r1)
li r30, 0
addi r29, r1, -0x960
@cmdloop
lbzx r31, r8, r30
stbx r31, r29, r30
addi r30, r30, 1
cmpwi r31, 0
bne @cmdloop
lwz r31, -0x404(r1)
stw r8, -0x404(r1)
bl panic
lwz r8, -0x404(r1)
li r0, 0
stw r0, 0(r8)
stw r31, -0x404(r1)
b ReturnZeroFromMPCall
@fail
b ReturnMPCallOOM
endif