英文:
Bit shift resulting in zero when it shouldn't
问题
我正在开发一个Chip8模拟器。Chip8指令以两个字节对的形式存储，我将其作为uint16处理。操作码的数据被编织到操作码本身中。例如，绘制精灵的操作码是0xDxyn，实际上操作码只是0xD000，最后三位是要绘制的x和y位置，n表示精灵的字节数。
在这个基础上，我一直在尝试使用简单的位移和掩码来提取这些数据，但是我得到的结果是0，而不是预期的值。例如：
//Opcode is = 0xD01F, of type uint16
x := int(c.Registers[((opcode >> 16) & 0x000F)])
y := int(c.Registers[((opcode >> 8) & 0x000F)])
size := int((opcode & 0x000F))
这应该解析为c.Registers[0]和c.Registers[1]，但最终都变成了c.Registers[0]。Size的值是正确的，为15。为什么会这样呢？
英文:
I'm developing a Chip8 emulator. Chip8 instructions are stored as two byte pairs, which I handle as a uint16. The data for the opcodes is interwoven into the opcode itself. For example the opcode to draw a sprite is 0xDxyn, where the opcode is actually just 0xD000, with the last three places being the x and y location to draw, and the n being how many bytes the sprite is.
This is mind, I've been trying to extract this data using simple bitshifting and masking, but I keep getting 0 instead of the expected values. For example:
//Opcode is = 0xD01F, of type uint16
x := int(c.Registers[((opcode >> 16) & 0x000F)])
y := int(c.Registers[((opcode >> 8) & 0x000F)])
size := int((opcode & 0x000F))
This should resolve to c.Registers[0] and c.Registers[1], instead both end up c.Registers[0]. Size comes out correctly 15. Why is this?
答案1
得分: 5
如果将一个16位数向右移动16位，你将把所有数据都移出。
要获取x，你需要向右移动8位，要获取y，需要向右移动4位。（记住：每个十六进制数字是4位）
英文:
If you shift a 16 bit number 16 places to the right you've shifted all of the data out.
To get x you need to shift 8 bits and to get y, 4 bits. (Remember: each hex digit is 4 bits)
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