Game Development Reference
In-Depth Information
C compilers had the ability to build different executables from the same code for dif-
ferent platforms.
While assembly language has a one-to-one correlation with machine code, C is com-
piled into object code. This is a translation from the C code to machine code for that
particular platform. Those object files are then linked with each other and other
libraries to form a final, platform-specific executable. This is one major difference
between C and assembly language. Assembly language is tied directly to the machine
architecture, while the same C program can be compiled onto multiple platforms
with completely different architectures.
Another huge difference between assembly language and C is that C is much easier
for a human to understand. As an example, here
s a bubble sort function implemen-
ted in 6502 Assembly, which is what the Atari 2600 and NES used:
'
;DOWNLOADED FROM: http://6502.org/source/sorting/bubble8.htm
;THIS SUBROUTINE ARRANGES THE 8-BIT ELEMENTS OF A LIST IN ASCENDING
;ORDER. THE STARTING ADDRESS OF THE LIST IS IN LOCATIONS $30 AND
;$31. THE LENGTH OF THE LIST IS IN THE FIRST BYTE OF THE LIST. LOCATION
;$32 IS USED TO HOLD AN EXCHANGE FLAG.
SORT8
LDY #$00
;TURN EXCHANGE FLAG OFF (= 0)
STY $32
LDA ($30),Y ;FETCH ELEMENT COUNT
TAX
; AND PUT IT INTO X
INY
;POINT TO FIRST ELEMENT IN LIST
DEX
;DECREMENT ELEMENT COUNT
NXTEL
LDA ($30),Y ;FETCH ELEMENT
INY
CMP ($30),Y ;IS IT LARGER THAN THE NEXT ELEMENT?
BCC CHKEND
BEQ CHKEND
;YES. EXCHANGE ELEMENTS IN MEMORY
PHA ; BY SAVING LOW BYTE ON STACK.
LDA ($30),Y ; THEN GET HIGH BYTE AND
DEY
; STORE IT AT LOW ADDRESS
STA ($30),Y
PLA
;PULL LOW BYTE FROM STACK
INY
; AND STORE IT AT HIGH ADDRESS
STA ($30),Y
LDA #$FF
;TURN EXCHANGE FLAG ON (= -1)
STA $32
CHKEND DEX
;END OF LIST?
BNE NXTEL
;NO. FETCH NEXT ELEMENT
BIT $32
;YES. EXCHANGE FLAG STILL OFF?
