@@ -89,10 +89,11 @@
 assoc(`nsrc',      `#SRC    ')
 assoc(`obey',      `OBEY    ')
 assoc(`over',      `OVER    ')
 assoc(`rdrop',     `RDROP   ')
 assoc(`rot',       `ROT     ')
+assoc(`tuck',      `TUCK    ')
 assoc(`rtosrc',    `R>SRC   ')
 assoc(`source_id', `SOURCE-I')
 assoc(`source',    `SOURCE  ')
 assoc(`space',     `SPACE   ')
 assoc(`srcid',     `SRCID   ')
@@ -108,17 +109,20 @@
 assoc(`_u2slash',  `U2/     ')
 assoc(`within',    `WITHIN  ')
 assoc(`off',       `OFF     ')
 assoc(`on',        `ON      ')
 assoc(`_and',      `AND     ')
+assoc(`_bic',      `BIC     ')
 assoc(`_or',       `OR      ')
 assoc(`_xor',      `XOR     ')
 assoc(`cr',        `CR      ')
 assoc(`_1minus',   `1-      ')
 assoc(`_1plus',    `1+      ')
 assoc(`_2slash',   `2/      ')
 assoc(`_2star',    `2*      ')
+assoc(`cells',     `CELLS   ')
+assoc(`chars',     `CHARS   ')
 assoc(`rfrom',     `R>      ')
 assoc(`zeq',       `0=      ')
 assoc(`zlt',       `0<      ')
 assoc(`is_lt',     `<       ')
 assoc(`is_eq',     `=       ')
@@ -153,10 +157,29 @@
 assoc(`decimal',   `DECIMAL ')
 assoc(`binary',    `BINARY  ')
 assoc(`dots',      `.S      ')
 assoc(`depth',     `DEPTH   ')
 assoc(`pick',      `PICK    ')
+assoc(`first',     `FIRST   ')
+assoc(`flags',     `FLAGS   ')
+assoc(`bns',       `BNS     ')
+assoc(`last1',     `LAST    ')
+assoc(`used',      `USED    ')
+assoc(`empty_bufs',`EMPTY-BU')
+assoc(`fbic',      `find-blo')
+assoc(`ba',        `block-ad')
+assoc(`bclr',      `BCLR    ')
+assoc(`bset',      `BSET    ')
+assoc(`snb',       `select-n')
+assoc(`wb',        `write-bl')
+assoc(`fb',        `flush-bl')
+assoc(`cb',        `configur')
+assoc(`lob',       `load-blo')
+assoc(`block',     `BLOCK   ')
+assoc(`update',    `UPDATE  ')
+assoc(`buffer',    `BUFFER  ')
+assoc(`flush',     `FLUSH   ')
 
 ; DX-Forth is intended to be loaded at address $14200, and not
 ; exceed $17FFF.  The dictionary space is intended to occupy
 ; from $18000-$1BFFF.  Somewhere in this space must also exist
 ; things like the Forth stacks and block buffers.
@@ -165,11 +188,12 @@
 ; so far:
 ;
 ; $13E80 - $13FFF	Return Stack (aka BIOS stack)
 ; $14000 - $141FF	BIOS State (opaque; do not touch!)
 ; $14200 - $17FFF	DX-Forth Image and state
-; $18000 - $1BDFF	User programs and state
+; $18000 - $1ADFF	User programs and state
+; $1AE00 - $1BDFF	Block I/O buffers
 ; $1BE00 - $1BFFF	Data Stack
 ;
 ; Note that the exact boundary between DX-Forth and the user
 ; dictionary space is fluid.  The above memory map assumes the
 ; worst-case scenario where DX-Forth occupies a solid 16KiB.
@@ -361,10 +385,16 @@
 	ld	a2,8(S)
 	sd	a1,8(S)
 	sd	T,0(S)
 	addi	T,a2,0
 	jal	x0,next
+
+; Tuck a datum away.  ( a b -- b a b )
+
+	align	4
+_tuck:	jal	W,_docol
+	hword	dup, rot, rot, exit
 
 ; Return stack manipulations.
 
 	align	4
 _rdrop:
@@ -485,10 +515,17 @@
 	ld	W,0(S)
 	addi	S,S,8
 	and	T,T,W
 	jal	x0,next
 
+	align	4
+__bic:	ld	W,0(S)
+	addi	S,S,8
+	xori	W,W,-1
+	and	T,T,W
+	jal	x0,next
+
 	align	4
 __or:	ld	W,0(S)
 	addi	S,S,8
 	or	T,T,W
 	jal	x0,next
@@ -519,10 +556,14 @@
 	align	4
 __2slash:
 	srai	T,T,1
 	jal	x0,next
 
+	align	4
+_cells:	slli	T,T,3
+_chars:	jal	x0,next
+
 	align	4
 __u2slash:
 	srli	T,T,1
 	jal	x0,next
 
@@ -1273,10 +1314,190 @@
 	align	4
 _dots:	jal	W,_docol
 	hword	dotdepth, dotvals, space, space, exit
 
 ; END PICTURED NUMERIC OUTPUT
+
+; BEGIN BLOCK I/O ============================================
+;
+; 4KiB of block I/O buffers sits at $1AE00 in this version.
+; This base address is answered by the word FIRST.
+;
+; Each buffer has a corresponding flag byte which indicates
+; whether the buffer has been assigned to a block, and/or
+; whether the buffer has been modified since it was assigned
+; last.  The private word FLAGS gives the address of the first
+; byte.
+;
+; Finally, each buffer has a corresponding block mapping.
+; The word BNS returns the address of the 4-vector of Block
+; Numbers.
+;
+; To help implement a crude approximation of a least-recently-
+; used algorithm, we need two additional variables.
+;
+; LAST refers to the last referenced buffer entry (0 <= LAST
+; < 4).  This is so UPDATE knows which buffer to mark as dirty.
+;
+; USED just cycles from buffer to buffer.  This round-robin
+; selection of buffers is not a true LRU, but for 99% of the
+; use-cases for using blocks, it gives a good enough facsimile.
+;
+; EMPTY-BUFFERS ( -- ) resets the block I/O system to its
+; power-on default state.  All buffers are marked unused.
+; Any data which has been modified is lost.
+;
+; NOTE: in this version of Forth, due to word length
+; limitations, you can get away with just EMPTY-BU.  Please
+; don't -- use the full name.  You'll thank me later when
+; more characters becomes significant in the future.
+;
+; BUFFER is identical to BLOCK in every way, except that it
+; does not fill the buffer from secondary storage.  The
+; assumption is that you'll just copy over the entire contents
+; of the buffer anyway (e.g., as when copying blocks around),
+; so why waste time filling the buffer contents when it's not
+; going to be used?
+;
+; FLUSH will flush all dirty buffers back to storage, and
+; mark them clean again.  However, it will not affect the
+; state of the cache as a whole (vs. EMPTY-BUFFERS).
+
+
+	align	4
+_first:	jal	W,_docon
+	align	8
+	dword	$1AE00		; First of four 1KiB blocks
+
+	; We use only four bytes of the space reserved for
+	; the flags "variable".  Each byte has the following
+	; bit meanings:
+	;
+	; .... ...1	Buffer has been assigned to a block.
+	; .... ..1.	Buffer has been modified since filled.
+	; 0000 00..	Must be zero.
+
+	align	4
+_flags:	jal	W,_dovar
+	align	8
+	dword	0
+
+	align	4
+_bns:	jal	W,_dovar
+	align	8
+	dword	0,0,0,0
+
+	align	4
+_last1:	jal	W,_dovar
+	align	8
+	dword	3
+
+	align	4
+_used:	jal	W,_dovar
+	align	8
+	dword	0
+
+	align	4
+_empty_bufs:
+	jal	W,_docol
+	hword	lit16, 0, flags, store
+	hword	bns, lit16, 4, cells, lit16, 0, fill
+	hword	lit16, 3, last1, store, exit
+
+	align	4
+_fbic:	jal	W,_docol
+	hword	lit16, 0
+_fbic_0:
+	hword	dup, lit16, 4, is_ge, zgo, _fbic_1
+	hword	drop, lit16, 0, exit
+_fbic_1:
+	hword	dup, flags, plus, cfetch, zgo, _fbic_2
+	hword	twodup, cells, bns, plus, fetch, is_eq
+	hword	zgo, _fbic_3
+	hword	nip, lit16, -1, exit
+_fbic_3:
+_fbic_2:
+	hword	_1plus, go, _fbic_0
+
+	align	4
+_ba:	jal	W,_docol
+	hword	lit16, 1024, mult, first, plus, exit
+
+	align	4
+_bclr:	jal	W,_docol
+	hword	tuck, cfetch, _bic, swap, cstore, exit
+
+	align	4
+_bset:	jal	W,_docol
+	hword	tuck, cfetch, _or, swap, cstore, exit
+
+	align	4
+_snb:	jal	W,_docol
+	hword	used, fetch, _1plus, lit16, 3, _and
+	hword	used, store, exit
+
+	align	4
+_wb:	jal	W,_docol
+	hword	used, fetch, cells, bns, plus, fetch
+	hword	squote
+	byte	19, "WRITING BACK BLOCK "
+	align	2
+	hword	type, dot, cr, exit
+
+	align	4
+_fb:	jal	W,_docol
+	hword	used, fetch, flags, plus, cfetch
+	hword	lit16, 2, _and, zgo, _fb_1
+	hword	wb, lit16, 2, used, fetch, flags, plus, bclr
+_fb_1:	hword	exit
+
+	align	4
+_cb:	jal	W,_docol
+	hword	lit16, 1, used, fetch, flags, plus, cstore
+	hword	used, fetch, cells, bns, plus, store, exit
+
+	align	4
+_lob:	jal	W,_docol
+	hword	squote
+	byte	14, "READING BLOCK "
+	align	2
+	hword	type
+	hword	used, fetch, cells, bns, plus, fetch, dot
+	hword	squote
+	byte	13, " INTO BUFFER "
+	align	2
+	hword	type
+	hword	used, fetch, ba, hex, udot, decimal, cr, exit
+
+	align	4
+_block:	jal	W,_docol
+	hword	fbic, zgo, _block_1
+	hword	dup, last1, store, ba, exit
+_block_1:
+	hword	snb, fb, cb, lob
+	hword	used, fetch, dup, last1, store, ba, exit
+
+	align	4
+_buffer:
+	jal	W,_docol
+	hword	fbic, zgo, _buffer_1
+	hword	dup, last1, store, ba, exit
+_buffer_1:
+	hword	snb, fb, cb
+	hword	used, fetch, dup, last1, store, ba, exit
+
+	align	4
+_update:
+	jal	W,_docol
+	hword	lit16, 2, last1, fetch, flags, plus, bset, exit
+
+	align	4
+_flush:	jal	W,_docol
+	hword	lit16, 0, used, store
+	hword	snb, fb, snb, fb, snb, fb, snb, fb, snb, fb, exit
+
+; END BLOCK I/O
 
 ; ==== Look stuff up in the dictionary.
 ;
 ; The dictionary is a mapping of word name to code field
 ; address (CFA).  The dictionary is expressed as a relational