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James Gregory 2013-12-31 11:38:48 +11:00
commit f658bfbb40
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04-09.md
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@ -86,34 +86,36 @@ long as we had assumed, but a long time nonetheless.
**LISTING 4.11 LST4-11.ASM**
; Times speed of memory access to Enhanced Graphics
; Adapter graphics mode display memory at A000:0000.
;
mov ax,0010h
int 10h; select hi-res EGA graphics
; mode 10 hex (AH=0 selects
; BIOS set mode function,
; with AL=mode to select)
;
mov ax,0a000h
mov ds,ax
mov es,ax ;move to & from same segment
sub si,si ;move to & from same offset
mov di,si
mov cx,800h ;move 2K words
cld
call ZTimerOn
rep movsw ;simply read each of the first
; 2K words of the destination segment,
; writing each byte immediately back
; to the same address. No memory
; locations are actually altered; this
; is just to measure memory access
; times
call ZTimerOff
;
mov ax,0003h
int 10h ;return to text mode
```nasm
; Times speed of memory access to Enhanced Graphics
; Adapter graphics mode display memory at A000:0000.
;
mov ax,0010h
int 10h; select hi-res EGA graphics
; mode 10 hex (AH=0 selects
; BIOS set mode function,
; with AL=mode to select)
;
mov ax,0a000h
mov ds,ax
mov es,ax ;move to & from same segment
sub si,si ;move to & from same offset
mov di,si
mov cx,800h ;move 2K words
cld
call ZTimerOn
rep movsw ;simply read each of the first
; 2K words of the destination segment,
; writing each byte immediately back
; to the same address. No memory
; locations are actually altered; this
; is just to measure memory access
; times
call ZTimerOff
;
mov ax,0003h
int 10h ;return to text mode
```
For comparison, let's see how long the same code takes when accessing
normal system RAM instead of display memory. The code in Listing 4.12,
@ -125,24 +127,26 @@ cycle-eater can *more than double* the execution time of 8088 code!
**LISTING 4.12 LST4-12.ASM**
; Times speed of memory access to normal system
; memory.
;
mov ax,ds
mov es,ax ;move to & from same segment
sub si,si ;move to & from same offset
mov di,si
mov cx,800h ;move 2K words
cld
call ZTimerOn
rep movsw ;simply read each of the first
; 2K words of the destination segment,
; writing each byte immediately back
; to the same address. No memory
; locations are actually altered; this
; is just to measure memory access
; times
call ZTimerOff
```nasm
; Times speed of memory access to normal system
; memory.
;
mov ax,ds
mov es,ax ;move to & from same segment
sub si,si ;move to & from same offset
mov di,si
mov cx,800h ;move 2K words
cld
call ZTimerOn
rep movsw ;simply read each of the first
; 2K words of the destination segment,
; writing each byte immediately back
; to the same address. No memory
; locations are actually altered; this
; is just to measure memory access
; times
call ZTimerOff
```
Bear in mind that we're talking about a worst case here; the impact of
the display adapter cycle-eater is proportional to the percent of time a