diff --git a/blk/001 b/blk/001
index c3b0e68..5e258be 100644
--- a/blk/001
+++ b/blk/001
@@ -11,3 +11,4 @@ MASTER INDEX
 620 Sega Master System Recipe
 650 AVR assembler             730 8086 assembler
 800 8086 boot code            830 PC/AT recipe
+850 Zilog SIO driver
diff --git a/blk/850 b/blk/850
new file mode 100644
index 0000000..676548c
--- /dev/null
+++ b/blk/850
@@ -0,0 +1,5 @@
+Zilog SIO driver
+
+Declarations at B851
+
+Driver load range at B852-BXXX.
diff --git a/blk/851 b/blk/851
new file mode 100644
index 0000000..523a4f7
--- /dev/null
+++ b/blk/851
@@ -0,0 +1,14 @@
+0x80 CONSTANT SIO_ACTL   0x81 CONSTANT SIO_ADATA
+0x82 CONSTANT SIO_BCTL   0x83 CONSTANT SIO_BDATA
+0x20 CONSTANT SIO_BUFSZ ( SZ-1 must be a mask )
+( Address in memory that can be used variables shared
+  with SIO native words. 4 bytes used. )
+CREATE SIO_MEM SYSVARS 0x70 + ,
+( Points to SIO buf )
+: SIO( SIO_MEM @ 2+ ;
+( Read buf idx Pre-inc )
+: SIOR> SIO_MEM @ ;
+( Write buf idx Post-inc )
+: SIOW> SIO_MEM @ 1+ ;
+( This means that if W> == R>, buffer is full.
+  If R>+1 == W>, buffer is empty. )
diff --git a/blk/852 b/blk/852
new file mode 100644
index 0000000..7b6fcf8
--- /dev/null
+++ b/blk/852
@@ -0,0 +1,15 @@
+( INT handler. Set RST 38 jump ) PC ORG @ 0x39 + !
+AF PUSH, BEGIN,
+SIO_ACTL INAi, ( RR0 ) 0x01 ANDi, ( is recv buf full? )
+IFZ, ( nope, exit ) A 0x20 ( CMD 4 ) LDri, SIO_ACTL OUTiA,
+    AF POP, EI, RETI, THEN,
+HL PUSH,
+HL SIOW> LDdi, A (HL) LDrr,
+HL DECd, (HL) CPr, ( W> == R> ? )
+IFNZ, ( buffer not full )
+    ( get wr ptr ) HL SIO( LDd(i),
+    L ADDr, IFC, H INCr, THEN, L A LDrr,
+    ( fetch/write ) SIO_ADATA INAi, (HL) A LDrr,
+    ( advance W> ) SIOW> LDA(i), A INCr,
+    SIO_BUFSZ 1- ANDi, SIOW> LD(i)A,
+THEN, HL POP, JR, AGAIN,
diff --git a/blk/854 b/blk/854
new file mode 100644
index 0000000..2175d30
--- /dev/null
+++ b/blk/854
@@ -0,0 +1,14 @@
+: (key)
+    ( inc then fetch )
+    [ SIOR> LITN ] C@ 1+ [ SIO_BUFSZ 1- LITN ] AND
+    ( As long as R> == W>-1, it means that buffer is empty )
+    BEGIN DUP [ SIOW> LITN ] C@ = NOT UNTIL
+    DUP [ SIO( LITN ] @ + C@ ( ridx c )
+    SWAP [ SIOR> LITN ] C! ( c )
+;
+: (emit)
+    ( As long at CTL bit 2 is low, we are transmitting. wait )
+    BEGIN [ SIO_ACTL LITN ] PC@ 0x04 AND UNTIL
+    ( The way is clear, go! )
+    [ SIO_ADATA LITN ] PC!
+;
diff --git a/blk/855 b/blk/855
new file mode 100644
index 0000000..b1e6616
--- /dev/null
+++ b/blk/855
@@ -0,0 +1,12 @@
+: _ [ SIO_ACTL LITN ] PC! ;
+: SIO$
+    H@ [ SIO( LITN ] ! 0 [ SIOR> LITN ] C!
+    1 [ SIOW> LITN ] C! ( write index starts one pos later )
+    [ SIO_BUFSZ LITN ] ALLOT
+    0x18 _ ( CMD3 )
+    0x24 _ ( CMD2/PTR4 ) 0b11000100 _ ( WR4/64x/1stop/nopar )
+    0x03 _ ( PTR3 ) 0b11000001 _ ( WR3/RXen/8char )
+    0x05 _ ( PTR5 ) 0b11001000 _ ( WR5/TXen/8char )
+    0x21 _ ( CMD2/PTR1 ) 0b00011000 _ ( WR1/Rx INT all chars )
+    (im1)
+;