Xilinx UG129 manual Implementing a Look-Up Table, Scratchpad RAM

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Chapter 5: Scratchpad RAM

NAMEREG s0, ram_data

NAMEREG s1, ram_address

CONSTANT ram_locations, 40	;	there are	64 locations
CONSTANT initial_value, 00	;	initialize to zero
LOAD ram_data, initial_value	;	load initial value
LOAD ram_address, ram_locations ;		fill from	top to bottom
ram_fill: SUB ram_address, 01	;	decrement	address
STORE ram_data, (ram_address)	;	initialize location
JUMP NZ, ram_fill	;	if not address 0, goto
	;	ram_fill

Figure 5-2:Indirect Addressing Initializes All of RAM with a Simple Subroutine

Implementing a Look-Up Table

The next few examples demonstrate both the flexibility of the scratchpad RAM and indirect addressing. The example code in Figure 5-3uses Scratchpad RAM as a look-up table (LUT) to convert four binary inputs to the equivalent hexadecimal character display on a 7-segment LED. The code reads four external switches, resulting in a binary value between 0000 and 1111. The PicoBlaze microcontroller converts each four-bit switch value into the equivalent hexadecimal character as displayed on a 7-segment LED. The scratchpad RAM holds the LED output patterns in the first 16 locations. The input switch value is the address input to the RAM.

CONSTANT switches, 00	;	read switch values at port 0
CONSTANT LEDs, 01	;	write 7-seg LED at port 1
; Define 7-segment LED pattern {dp,g,f,e,d,c,b,a}
CONSTANT LED_0, C0	;	display '0' on 7-segment display
CONSTANT LED_1, F9	;	display '1' on 7-segment display
;
CONSTANT LED_F, 8E ; display 'F' on 7-segment display
NAMEREG s0, switch_value	;	read switches into register s0
NAMEREG s1, LED_output	;	load LED output data in register s1
; Load 7-segment LED patterns into scratchpad RAM
LOAD LED_output, LED_0	; grab LED pattern for switches = 0000
STORE LED_output, 00	; store in RAM[0]
LOAD LED_output, LED_1	; grab LED pattern for switches = 0001
STORE LED_output, 01	; store in RAM[1]
;
LOAD LED_output, LED_F	; grab LED pattern for switches = 1111
STORE LED_output, 0F	; store in RAM[F]
; Read switch values and display value on 7-segment LED
loop: INPUT switch_value, switches	; read value on switches
AND switch_value, F0	; mask upper bits to guarantee < 15
FETCH LED_output, (switch_value) ; look up LED pattern in RAM
OUTPUT LED_output, LEDs	; display switch value on 7-segment LED
JUMP loop

Figure 5-3:Using Scratchpad RAM as a Look-Up Table

46	www.xilinx.com	PicoBlaze 8-bit Embedded Microcontroller
		UG129 (v1.1.2) June 24, 2008