August 10th, 2017
0 reactions

The Alpha AXP, part 4: Bit 15. Ugh. Bit 15.

Raymond Chen
Raymond Chen

Let’s load each of the powers of two up to 2³¹, in canonical form. 

    LDA  Ra, 0x0001(zero)   ;   Ra = 0x00000000`00000001 ; bit  0
    LDA  Ra, 0x0002(zero)   ;   Ra = 0x00000000`00000002 ; bit  1
    LDA  Ra, 0x0004(zero)   ;   Ra = 0x00000000`00000004 ; bit  2
    LDA  Ra, 0x0008(zero)   ;   Ra = 0x00000000`00000008 ; bit  3
    LDA  Ra, 0x0010(zero)   ;   Ra = 0x00000000`00000010 ; bit  4
    LDA  Ra, 0x0020(zero)   ;   Ra = 0x00000000`00000020 ; bit  5
    LDA  Ra, 0x0040(zero)   ;   Ra = 0x00000000`00000040 ; bit  6
    LDA  Ra, 0x0080(zero)   ;   Ra = 0x00000000`00000080 ; bit  7
    LDA  Ra, 0x0100(zero)   ;   Ra = 0x00000000`00000100 ; bit  8
    LDA  Ra, 0x0200(zero)   ;   Ra = 0x00000000`00000200 ; bit  9
    LDA  Ra, 0x0400(zero)   ;   Ra = 0x00000000`00000400 ; bit 10
    LDA  Ra, 0x0800(zero)   ;   Ra = 0x00000000`00000800 ; bit 11
    LDA  Ra, 0x1000(zero)   ;   Ra = 0x00000000`00001000 ; bit 12
    LDA  Ra, 0x2000(zero)   ;   Ra = 0x00000000`00002000 ; bit 13
    LDA  Ra, 0x4000(zero)   ;   Ra = 0x00000000`00004000 ; bit 14
    LDAH Ra, 0x0001(zero)   ; \
    LDA  Ra, 0x8000(Ra)     ; / Ra = 0x00000000`00008000 ; bit 15
    LDAH Ra, 0x0001(zero)   ;   Ra = 0x00000000`00010000 ; bit 16
    LDAH Ra, 0x0002(zero)   ;   Ra = 0x00000000`00020000 ; bit 17
    LDAH Ra, 0x0004(zero)   ;   Ra = 0x00000000`00040000 ; bit 18
    LDAH Ra, 0x0008(zero)   ;   Ra = 0x00000000`00080000 ; bit 19
    LDAH Ra, 0x0010(zero)   ;   Ra = 0x00000000`00100000 ; bit 20
    LDAH Ra, 0x0020(zero)   ;   Ra = 0x00000000`00200000 ; bit 21
    LDAH Ra, 0x0040(zero)   ;   Ra = 0x00000000`00400000 ; bit 22
    LDAH Ra, 0x0080(zero)   ;   Ra = 0x00000000`00800000 ; bit 23
    LDAH Ra, 0x0100(zero)   ;   Ra = 0x00000000`01000000 ; bit 24
    LDAH Ra, 0x0200(zero)   ;   Ra = 0x00000000`02000000 ; bit 25
    LDAH Ra, 0x0400(zero)   ;   Ra = 0x00000000`04000000 ; bit 26
    LDAH Ra, 0x0800(zero)   ;   Ra = 0x00000000`08000000 ; bit 27
    LDAH Ra, 0x1000(zero)   ;   Ra = 0x00000000`10000000 ; bit 28
    LDAH Ra, 0x2000(zero)   ;   Ra = 0x00000000`20000000 ; bit 29
    LDAH Ra, 0x4000(zero)   ;   Ra = 0x00000000`40000000 ; bit 30
    LDAH Ra, 0x8000(zero)   ;   Ra = 0xFFFFFFFF`80000000 ; bit 31

Bit 15. Ugh. Bit 15.

It’s the only one that requires two instructions to calculate the 32-bit mask in canonical form.

This means that if you have a bitfield, bit 15 is slightly more expensive than the others.

Bonus chatter: In an internal discussion of byte manipulation on the Alpha AXP, I happened to have written “Bit 15. Ugh. Bit 15.” Gov Maharaj (co-host of The Defrag Show and application compatibility master extraordinaire) told me, “That needs to be the title of a blog post.” So here it is.

Category
Old New Thing
Topics
Code

Author

Raymond Chen

Raymond has been involved in the evolution of Windows for more than 30 years. In 2003, he began a Web site known as The Old New Thing which has grown in popularity far beyond his wildest imagination, a development which still gives him the heebie-jeebies. The Web site spawned a book, coincidentally also titled The Old New Thing (Addison Wesley 2007). He occasionally appears on the Windows Dev Docs Twitter account to tell stories which convey no useful information.

0 comments

Discussion is closed.

Read next