diff --git a/blk/065 b/blk/065
new file mode 100644
index 0000000..1de4477
--- /dev/null
+++ b/blk/065
@@ -0,0 +1,16 @@
+( ATmega328P definitions ) : > CONSTANT ;
+0xc6 > UDR0 0xc4 > UBRR0L 0xc5 > UBRR0H 0xc2 > UCSR0C
+0xc1 > UCSR0B 0xc0 > UCSR0A 0xbd > TWAMR 0xbc > TWCR
+0xbb > TWDR 0xba > TWAR 0xb9 > TWSR 0xb8 > TWBR 0xb6 > ASSR
+0xb4 > OCR2B 0xb3 > OCR2A 0xb2 > TCNT2 0xb1 > TCCR2B
+0xb0 > TCCR2A 0x8a > OCR1BL 0x8b > OCR1BH 0x88 > OCR1AL
+0x89 > OCR1AH 0x86 > ICR1L 0x87 > ICR1H 0x84 > TCNT1L
+0x85 > TCNT1H 0x82 > TCCR1C 0x81 > TCCR1B 0x80 > TCCR1A
+0x7f > DIDR1 0x7e > DIDR0 0x7c > ADMUX 0x7b > ADCSRB
+0x7a > ADCSRA 0x79 > ADCH 0x78 > ADCL 0x70 > TIMSK2
+0x6f > TIMSK1 0x6e > TIMSK0 0x6c > PCMSK1 0x6d > PCMSK2
+0x6b > PCMSK0 0x69 > EICRA 0x68 > PCICR 0x66 > OSCCAL
+0x64 > PRR 0x61 > CLKPR 0x60 > WDTCSR 0x3f > SREG 0x3d > SPL
+0x3e > SPH 0x37 > SPMCSR 0x35 > MCUCR 0x34 > MCUSR 0x33 > SMCR
+0x30 > ACSR 0x2e > SPDR 0x2d > SPSR 0x2c > SPCR 0x2b > GPIOR2
+0x2a > GPIOR1 0x28 > OCR0B 0x27 > OCR0A 0x26 > TCNT0  ( cont. )
diff --git a/blk/066 b/blk/066
new file mode 100644
index 0000000..3c6f9eb
--- /dev/null
+++ b/blk/066
@@ -0,0 +1,6 @@
+( cont. ) 0x25 > TCCR0B 0x24 > TCCR0A 0x23 > GTCCR
+0x22 > EEARH 0x21 > EEARL 0x20 > EEDR 0x1f > EECR
+0x1e > GPIOR0 0x1d > EIMSK 0x1c > EIFR 0x1b > PCIFR
+0x17 > TIFR2 0x16 > TIFR1 0x15 > TIFR0 0x0b > PORTD 0x0a > DDRD
+0x09 > PIND 0x08 > PORTC 0x07 > DDRC 0x06 > PINC 0x05 > PORTB
+0x04 > DDRB 0x03 > PINB
diff --git a/doc/asm.txt b/doc/asm.txt
index 76a54ba..08e850e 100644
--- a/doc/asm.txt
+++ b/doc/asm.txt
@@ -217,6 +217,22 @@ words expecting branching wordref as an argument. Examples:
 L2 ' BRTS FLBL! ( branch forward to L2 )
 L1 ' RJMP LBL, ( branch backward to L1 )
 
+# Model-specific constants
+
+Model-specific constants must be loaded separately. Here is a
+list of units:
+
+- ATMega328P: B65-B66
+
+Those units contain register constants such as PORTB, DDRB, etc.
+Unlike many moder assemblers, they do not include bit constants.
+Here's an example use:
+
+DDRB 5 SBI,
+PORTB 5 CBI,
+R16 TIFR0 IN,
+R16 0 ( TOV0 ) SBRS,
+
 # AVR instructions list
 
 OPRd (B53)