powermac-rom/NanoKernel/NKAdditions.s
Elliot Nunn 4c0e5221f1 Remove unnecessary static branch hints
These + and - characters in a branch mnemonic can cause the assembler to
produce conditional branch instructions with that hint the branch
predictor. The default for forward branches is -, and for backward
branches is +. If a mnemonic is issued with the opposite sign, then bit
10 of the instruction (the LSB of the BO field) is set.

My long-retired "ppcdisasm" script inserted these hints unconditionally,
despite 98% of them not being required. The code is much cleaner now.

I read in some old MPW release notes that PPCAsm and PPCLink together
exhibit a quirk when linking conditional branches to imported symbols.
PPCAsm always assembles these conditional branches as if they were
forward branches (that is, a + hint will always set the hint bit, and a
- hint will never). I hoped to use this property to divine whether the
NanoKernel was linked from one or many assembly files, but I was
frustrated by the lack of conditional branches between files.
2018-04-15 16:52:14 +08:00

62 lines
1.1 KiB
ArmAsm

;_______________________________________________________________________
; My additions to the NanoKernel, to go at the end of the code image
;_______________________________________________________________________
if &TYPE('NKDebugShim') != 'UNDEFINED'
DeclareMPCall 200, NKDebug
NKDebug
; Lifted from NKxprintf:
; Put the physical address of the r3 arg in r8
rlwinm. r9, r11, 0, MSR_DRbit, MSR_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