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more portable signed byte handling

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This commit is contained in:
Jorj Bauer 2019-12-14 21:54:28 -05:00
parent b584954d57
commit ee9898006c
1 changed files with 8 additions and 4 deletions
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12
6502.lua
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@ -1,8 +1,7 @@
local string = require "string"
local _ENV = require 'std.normalize' {
'std.strict',
'const',
-- 'string', -- this neuters string.pack and string.unpack, not sure why.
'string',
'io',
}
@ -505,7 +504,7 @@ _M.getParam = {
-- Have to convert this from an unsigned byte to a signed byte.
-- Values are from -128 to +127.
local p = string.unpack("b", string.pack("B", self:readmem(self.pc)))
local p = self:unsignedToSigned(self:readmem(self.pc))
self.pc = (self.pc + 1) & 0xFFFF
p = (p + self.pc) & 0xFFFF
return p
@ -516,7 +515,7 @@ _M.getParam = {
self.pc = (self.pc + 1) & 0xFFFF
-- Again, have to convert from unsigned byte to signed byte
local zprelParam2 = string.unpack("b", string.pack("B", self:readmem(self.pc)))
local zprelParam2 = self:unsignedToSigned(self:readmem(self.pc))
self.pc = (self.pc + 1) & 0xFFFF
zprelParam2 = (zprelParam2 + self.pc) & 0xFFFF
@ -1403,4 +1402,9 @@ function _M:writemem(a,v)
self.ram[a] = v
end
function _M:unsignedToSigned(v)
if ((v & 0x80) == 0x00) then return v end
return -((v ~ 0xFF) + 1)
end
return _M