On 08/09/2015 01:13 PM, Laurent Vivier wrote:
m68k: allow to update flags with operation on words and bytes
m68k: update CPU flags management
m68k: add X flag helpers
I wonder if we can talk about a different mechanism for tracking flags.
The x86 scheme tracks flags with 3 words: { op, dest, src }. This is easy to
handle arithmetic, logicals, and comparisons. But shifts and other weird
things require complex helpers.
The m68k scheme, which you're enhancing, is the same, except with an additional
word for X. It's this extra X flag that complicates things.
The arm scheme tracks 4 flags with 4 individual words. This is more
complicated for arithmetic (especially computing overflow), but it rarely
requires complicated helpers.
I propose using a hybrid between the two, using 6 words: { op, c, v, z, n, x }.
op = OP_FLAGS (i.e. fully computed state)
X = cc_x is 0/1
C = cc_c is 0/1
V = cc_v < 0
N = cc_n < 0
Z = cc_z == 0
op = OP_ADD
X = C = cc_x is 0/1
cc_n = result
cc_v = src1
cc_c = unused
cc_z = unused
N = cc_n < 0
Z = cc_n == 0
V = ((result ^ src1) & ~((result - src1) ^ src1)) < 0
op = OP_SUB
like OP_ADD, except
V = ((result ^ src1) & (src1 ^ (src1 - result))) < 0
op = OP_LOGIC
X = cc_x is 0/1
C = 0
V = 0
cc_n = result
N = cc_n < 0
Z = cc_n == 0
All of the byte and word variants are handled by sign-extending the quantities.
This keeps X up-to-date at all times. It minimizes the amount to data that
needs to be synced back to env before memory operations. It leaves enough data
to easily compute signed comparisons.
The more complicated cases of addx and shifts, now simply operate in OP_FLAGS
mode and don't require helpers.
I'll follow up with some patches...
r~
