Math

The eZ8 has some very basic math instructions: Add, subtract, increment, decrement, compare, and multiply. The eZ8 does not have an instruction for Division. Instead use a Division Routine. There also your standard bitmath instructions: And, Or, exclusive Or, Complement(not), Rotate, Shift, Bit set.

In this article the regular math instructions will be covered.

Addition

Assembly Mnemonic Symbolic Operation
ADC dst,src dst ← dst + src + C
ADCX dst,src allows for extended addressing
   
ADD dst,src dst ← dst + src
ADDX dst,src allows for extended addressing

ADD adds two values. To use it:

add R0,R1; R0 = R0 + R1
add R0,#5; R0 = R0 + 5

ADC adds two values + carry. Use it to add 16 bit numbers for example. To use it:

;Add an 8 bit number to a 16 bit value.
;First add the lower bytes and then add 0 +carry to the higher byte
add R1,R3; Notice the order of the additions!
adc R0,#0; R1 is the lower byte or register pair RR0

;Add an 8 bit value to a 16 bit value.
;First add the lower bytes and then add the higher bytes with carry.
add R1,R3 
adc R0,R2

Subtraction

Assembly Mnemonic Symbolic Operation
SBC dst,src dst ← dst - src - C
SBCX dst,src allows for extended addressing
   
SUB dst,src dst ← dst - src
SUBX dst,src allows for extended addressing

SUB subtracts two values. To use it:

sub R0,R1; R0 = R0 - R1
sub R0,#5; R0 = R0 - 5

SBC subtracts two values - carry. Use it to subtract 16 bit numbers for example. To use it:

;Subtract an 8 bit number from a 16 bit value.
;First subtract the lower bytes and then subtract carry from the higher byte.
sub R1,R3; Notice the order of the subtractions!
sbc R0,#0; R1 is the lower byte or register pair RR0

;Subtract a 16 bit value from a 16 bit value.
;First subtract the lower bytes and then subtract the higher bytes with carry.
sub R1,R3 
sbc R0,R2

Increment/decrement

Assembly Mnemonic Symbolic Operation
INC dst dst ← dst + 1
INCW dst same but for a register pair
   
DEC dst dst ← dst - 1
DECW dst same but for a register pair

INC/INCW increments register value by 1. To use it:

;increment register R2
inc R2
;increment register pair RR0
incw RR0

DEC/DECW decrements register value by 1. To use it:

;decrement register R2
dec R2
;decrement register pair RR0
decw RR0

Comparisson

Comparing two values is like subtraction but without affecting any registers other than the flags register. By looking at the flags you can see if the compared values are equal or unequal, greater or smaller. The Jump Instructions check the flags if you specify a condition code.

Assembly Mnemonic Symbolic Operation
CP dst,src dst - src
CPX dst,src allows for extended addressing
   
CPC dst,src dst - src - C
CPCX dst,src allows for extended addressing

Multiplication

The eZ8 has a multiplication instruction which is really helpful. Not all cpu have a multiplication instruction because it requires a lot of transistors.

Assembly Mnemonic Symbolic Operation
MULT dst dst[15..0] ← dst[15..8] * dst[7..0]

MULT multiplies the lower by the higher byte from a register pair and stores it back in the register pair. To use:

ld R0,#37
ld R1,#20
mult RR0 ; Multiplies R0 by R1 and stores the 16 bit result in pair RR0
; in this case the result is 740
; register R1 contains the lower portion of this number
; register R0 contains the HIGHER portion of this number

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